It's a gag, and one you both (MFauli and shinta) are taking far too seriously.

Do you think a character's face transforms into a Joestar or Kenshiro when they parody those? Do girls eyes go completely white when they give an homage to Glass Mask?

It's a gag.

Fujimiya's plumper face, contorted expression from a sneer, and her hideous bowl haircut from elementary school is obviously exaggerated for effect. It explains why her middle school femininity and now college age sexual appeal doesn't really work on Takefumi. She's liked him the whole time. She bullied him because that's what stupid girls do to the boys the like. The exaggerated features emphasize that.

She didn't get plastic surgery, wtf is wrong with the both of you?! It's stylistic.

When the Uncle described how everyone in the otherworld is exceptionally attractive by our standards, the art leveled up to show it and the bandits turned into ikemen for a flash. The otherworlders don't morph back and forth.

If you personally don't find it funny, I'm leaving that up to cultural differences in humor. We encounter those issues all the time across many series.