I really like this idea! I think a "Don't display this critique request anymore" toggle would be excellent - it doesn't really matter why out of all the possible reasons, but if you've chosen not to critique a piece being able to just not see it anymore can be helpful. I'm personally in favor of this. :) (Disclaimer: this is a personal endorsement only, Kim is the person who decides whether features get on the list or not!)
I do think that it's a good idea to try and give your input even if you feel you don't have a lot of experience with the subject at hand. Sometimes encouragement can be helpful as well and just what a person needs to improve themselves, so don't discredit yourself too quickly! I also recognize that at some point, everything that can be said has been said on an image, which is why I'd like to see this feature as well.