Hello @LucasTrusted and a very warm welcome to the Community Forum 
In answer to your question, no it is not mandatory for pet parents to leave reviews or for sitters to leave feedback. However, I would strongly advise that pet parents leave reviews and sitters leave feedback following a sit.
This not only helps the sitter to secure future sits, it also allows future pet parents to see any issues that have arisen and then they can choose their sitter/s accordingly making an informed decision. The same goes for pet parents, by sitters leaving feedback, future sitters can then make an informed choice as to whether they would still like to proceed with their application following any points that have been highlighted but also when feedback is left, this is a good sign for potential sitters, reassuring them that things have gone well and therefore encouraging them to apply rather than being overlooked because of possible red flags that are often associated when no feedback is left at all. I hope this helps.
For reference, as a member, you can also add your TrustedHousesitters profile on to your Forum profile by following the attached link, should you wish. This will then enable others to offer you helpful advice and feedback.
If you are unsure how to do this, or if you have any questions at all, do not hesitate to ask, we are all more than happy to help.
Also, remember you can use the spyglass symbol at the top of the page next to the TrustedHousesitters logo and search for any questions that you may have as they have most likely already been answered on here and of course if not then please just ask! 