The oven is the perfect example I reckon, something spills, it’s too hot to clean it, then it goes out of your mind. We do it at home, put the oven on, it smokes and you think damn I missed a spill. We had Airbnb’s where the toaster crumb catcher is out on display on top of the toaster when we arrive. That reminds me, clean the toaster before I leave
because I am soooo happy to have a toaster I always oblige.
Our simple moto is to leave a home “as clean” if not “cleaner” than we found it. To try and define cleanliness will only lead to madness IMHO.
@Cathie Exactly my thoughts to @avidtraveler earlier 
I follow the HO’s lead on what they consider clean. If the kitchen is sparkling when I arrive, I make the extra effort to leave it sparkling - including sponging down the microwave, toaster oven, etc. if I’ve used them. If the HO leaves the house tidy but not really clean, I don’t bend over backwards to vacuum all the dust bunnies that were in the corners when I arrived. Many homes I’ve sat in are comfortably lived in, which I appreciate because then I’m not exhausted by too much cleaning before I leave. By the way, I’ve gotten all 5-star marks for cleanliness because I pay attention to the details that matter and am very germ-conscious so I disinfect whatever bathrooms I’ve used and the kitchen sink. If I read host reviews that indicate too high expectations, such as laundering the bedding before I leave, I don’t apply to those sits. I usually stay a day’s drive from home and can’t imagine having to wash and dry sheets and towels before leaving the sit! I ask them before the sit what they’d like me to do with the linens, and luckily no one has asked me to launder the linens and remake my bed!
As for the OP, I think it’s gross to have to use an appliance that has ick on it, so I think giving them one less star for cleanliness is fair. It doesn’t mean a four-star sit overall if you were happy with the pet care and communication, etc. Although I steer clear of HOs who expect deep cleanings, if you explained briefly in your review that overall the house was tidy but the sitter left appliances dirty, you would not come across unreasonable to me. Hope all if our input helps!
I don’t dust.
Clearly you’re not a 5 star sitter then 
In my opinion, it’s not petty. But that’s just my opinion, clearly others don’t agree. I try to adjust my review a bit to what seems generally expected by THS community but still keep it honest and clear. IMO, the star system doesn’t really work here but the wording is very important.
What you could do is keep 5* but mention something like
If you gave 4* with explanations I would also consider it reasonable and it wouldn’t keep me from applying. It would also help me bear in mind to pay extra attention to the aspects you mention.
Regarding the general question, for me
But you can add a check list to your Welcome Guide
If a host complained about someone forgetting to clean the oven, I wouldn’t apply for that sit.
This past week I was upfront in a WhatsApp chat with a HO- I said that I didn’t do yukky houses, and by yukky that I meant kitchens/bathrooms etc. I also said I was mentioning it in case she wondered about me putting that sentence in my profile and that people had told me it might put HOs off, to which I had replied ‘good!’ Anyway the HO laughed and said she’d been a surgical ward sister for 30 years so there wouldn’t be a problem! I got the sit confirmed folks. 
Be clear about your expectations and don’t avoid saying them. Good HOs will not shy away from you. Of course if you don’t care that much you’d better say that too so that the HO knows upfront your style of living as it might suit them.
Once again, THS is about good matches, I am with @BonnyinBrighton, it’s good to be avoided if this saves me from a bad experience.
Regarding ovens and other appliances, I have a double standard, I am more demanding on myself than the host, meaning, it’s their home, so their standard. I have found quite a few dirty ovens in a house which was otherwise spotless and had not mentioned it in the review. I wasn’t bothered by that at all but I would not consider it petty if a HO mentioned it in their review. In fact I would preferred to be warned about all the aspects they consider important. If I don’t like the tone, I won’t apply but for me, it’s more about the tone than stating facts.
From a host’s perspective, perhaps they need to consider how big their pool of applicants is and if they can afford losing a few in order to improve the chances of getting great matches or if they may risk looking for alternative pet care. If that is the case, then maybe it’s worth paying for someone to clean the oven and have happy pets in their own home. But there’s nothing wrong with asking for both.
Homes and appliances are as varied as the hosts themselves so it would be difficult for THS to define a generic standard that covers everything.
For example one week into a sit - we noticed small flies in the kitchen coming from the fancy coffee machine . On investigation the coffee grounds had not been emptied by the host and since we aren’t coffee drinkers we hadn’t been using the machine . We cleaned it . We didn’t comment or mark the hosts down because if we had been using the machine daily as they did it would have been apparent much earlier and not a problem .
Our first sit had an Aga stove and the welcome guide contained information such as please wipe up any spills straight away because the heat of the Aga bakes them on and it is very difficult to remove . This was helpful and ensured that we used it appropriately.
I lived in a shared flat. My roomie was very upset that I did not wash the oven pan after using it. I had never in my life done that, and I had been living alone for 10 years. Also I always used a paper lining. But my roomie had been brought up always cleaning it.
From this I learned a lot. (Never started to wash the pan at my own home). I would love a host to make a point by point list of what they want me to do. Other sitters might find that annoying or too much. But I would prefer - I will not clean your toaster because I assume you’re normal clean level, if you are not, write it down. I make my own list when I sit, but of course I can not know if it is good enough for the host.
I get 5 stars in cleanliness usually. And I get mentions of having left the flat very clean. So I am doing something right. However I think some hosts think that I mop the floors or something, which I never do. I dont even vacuum (I prefer to use a broom). I spot clean, I clean the visual areas. I dont really deep clean. So the host will be happy and satisfied, but they might feel like I “cheated” if they knew how I did it. Cleaning is a lot about optics, using products that leave a smell, to prove they have been used (for my perspective a smelly product makes it dirty, not cleaner, so yes, cleanness is a perspective thing). I take pictures of appliances so I know how neurotic I need to be in the end.
I dont choose hosts who have docked stars in cleanliness because I fear that I will struggle to reach their expectations.
A while ago I recommended THS to an acquaintance who is an absolutely lovely person, but whose house is, quite frankly, a bit of a mess. (She had a lot going on at the time, which didn’t help.) I was wondering how I should broach this without hurting her feelings, as to be honest, I know I would find it disheartening to stay there in its current state, and I’m sure that other sitters would feel the same.
Luckily, she asked me whether I thought she’d need to tidy and clean in anticipation of a sitter’s arrival. It gave me the opportunity to explain that from a sitter’s point of view, a sit can be a bit of a change of scene, a working holiday, and an escape from the daily grind, and that yes, it would be far nicer to arrive to somewhere that gives that feeling of escapism, rather that the sinking feeling that accompanies having to dig out the Marigolds before you’ve even started. Perhaps rather than trying to define ‘cleanliness’, more emphasis should be placed upon this sentiment.
Homeowners really have the opportunity to set the tone on cleanliness, depending on how clean and tidy they leave their home for a sitter, and should definitely expect their home to be returned to them in the same state.
I did a sit and it looked spotless when I walked in but the oven hadn’t been cleaned in a very long time. When I tried to broil something, it smoked so bad I had to turn it off and couldn’t use it. I have been in homes where the toaster oven and oven had not been cleaned prior to my sitting. I do not clean them for the HOs. And I boil water to make my own coffee with a cup drip. I try and keep a small footprint. One, I sit in some very large homes. I do not want to clean, what I do not use, so I close off rooms. I have cleaned cupboards, in the beginning, since I could not stand the stickiness, though they looked clean enough. Everyone has a different version of clean.
Dusting, yes, I don’t dust, unless it was a really long sit and there is obvious where I used. But sometimes there is so many tchotchkes, et al. I don’t dust those.
Clearly not. 
But, if you want a home dusted get a cleaner.
me neither.
I am 2 days from the end of an 18 day sit in a house with quite the dirtiest, grimiest bathroom I have ever come across in 54 sits. I have never marked anyone down on cleanliness,
I wrote a post about it the other day, with photos. absolutely zero effort had been made to clean shower area, and they will be getting a 3 out of 5 on cleanliness from me.
AFTER the reviews are in, I will share with them my before and after photos.
I have shown the photos to some friends, one of them a sitter, one of them a host, they both said they would have walked away. the HO friend, even suggested that I give them a 1 for cleanliness, but I wont do that, as bedroom, living room and kitchen are fine.
So to @avidtraveler avidtravellers point, I do think CLEANLINESS requires more definition, i.e. bathroom should be cleaned to a hygienic standard.
Before our sitters arrive we spend several days cleaning the whole house: vacuuming, dusting, washing the floors, washing the appliances, cleaning the kitchen and bathroom, cleaning the litters area and washing the litter boxes, etc.
When we come back we do not expect all that. We just expect the kitchen (countertops, sink, dishes) and bathroom (toilet, bath, shower, countertops) to be cleaned. We prefer to do the rest ourselves when we come back. We want the sitters to be able to enjoy time with our cats and prepare their things without rushing for a deep clean. 
I agree with you on this. I don’t expect the sitters to do what I do and I never expect a sitter to launder the sheets and towels. I even tell them up front not to do this, I’m retired so I have a lot of time. It just bothered me that when I went to use the oven, it started smoking and smelling up the house. I’ve had 10 sits and this has happened twice now.
I get the sitter may have forgotten to clean it up after it’s cooled off.