I feel as though there should be some sort of "I don't feel comfortable critiquing this artwork" or "this has already recieved sufficient critique" button to make it go away for you personally - Because now I sit and skip images people have already raised the ways they can improve and I don't want to echo what has been said or just say good job! to get the images out of my critique section. It's not really that big of an issue but just a thought
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.
I will add this to my list of stuff to work on down the line :)
In reference to not wanting to "just" say good job - I would invite you to reconsider not wanting to do that, whether or not this feature goes into effect. Feeling your efforts are seen can be just as motivating and contribute to improvement as actual critique, especially if you are new to receiving critique and need the extra fuel to keep going even as things you are doing wrong are pointed out to you. :)