He likes seeing her mess up and stuff, so when he compliments her clothes, its a bit funny to imagine that he likes them, too, because he thinks they're horrible and that she has horrible taste in clothing. (Especially considering the fact that her outfit's really good here.)
Actually, what if the reason he hasn't noticed her at his school is because he can't recognize her when she's not in constant-failure mode? Her clothes are normal and she's not doing anything too silly so he's just like "huh, that's a normal girl..."