It's my first idea, so bear with me.
I know that mirrors are ridiculously over used, and I don't mean to copy anyone.
The basic idea is that it is a vanity set that causes onlookers to view themselves with disgust. It lowers their self esteem after prolonged exposure to the point of unrelenting suicidal tendencies.
I was thinking SCP-336 "Lilith" might give an interesting test log. As Lilith, when looking at herself in a mirror, fawns over herself. This would do quite the opposite.
It's a basic idea and I don't want to over think it until I get some feedback.