Do people get banned for negative comments?
I would think that people feel inclined to leave positive comments because 1) they honestly feel that way, 2) it can be scary to put yourself out there as only an image and people can empathize with that fear of judgment, and 3) many people are already mean enough to themselves, which means you're not offering the other person much if you don't have anything positive to say and decide to anyway.
That said, if I ever post a selfie, you all can feel free to be honest, however you'd like.
P.S.: That really is a nice selfie, Koppai. Your eyes appear to be a lovely glacial blue that the lighting and photo are capturing really well.