I think the person got 30+ reps because other TSR users could relate to the person. So all the people that think they're unattractive themselves, repped the person that said ''I think I'm unattractive blah blah''. Whereas, they negged you because they either thought you were being vain (maybe they thought you said it in the wrong way), or because they're jealous and they can't relate to your comment.
There's nothing wrong with thinking you're attractive at all, but if, for example, you say it in a snobby way like ''I know I'm attractive, many guys have complimented me, they've asked me out countless of times blah blah'', then you're expecting to get called vain and stuff tbh. The unattractive people want the attractive people to stay humble.
Plus, this is TSR. TSR doesn't represent society at all. People IRL don't ask others if they think consider themselves attractive LOL. Nobody has that kind of conversation IRL.