They're okay when the subject of the joke plays along with the role and reacts accordingly, like Rushia, but feel forced when the subject doesn't actually give a shit, like Gura. There's no point in teasing someone for being flat when they don't mind the fact that they're flat, but people have been conditioned by harem and romcom anime to think that any non-loli female character with a flat chest must resent the fact that she's flat.
This happens to anime characters too, like Tsubasa from Symphogear who gets a ton of fanart about resenting her flat chest even though she never once acted like that in the show.
