Maybe it's me but how & where can I leave comments regarding a sitter?

You are only able to leave a review after you have booked with that sitter and after your reservation is over, Rover will send you an email asking that you leave a review. If you want to leave a testimony for a sitter, that sitter would have to send you a request.

If you have concern about care provided by your Rover sitter you should contact Rover support at [Edit: Rover’s contact options have changed. Visit the Rover Help Center at https://support.rover.com/ to find the phone number, help articles, or chat with the team]