Yozora, the "snarky" character, is just mean and misanthropic.
Sena, the perfect superhuman school idol, is self-centered, near-universally disliked for it, and is socially incompetent as a result.
Sena and Rika are otaku surrogates, and the others consider the otaku behavior to be creepy.
Yukimura, the guy who looks weirdly girly, really is a girl, and her gender confusion says disquieting things about the way she was raised.
Kobato, the chuunibyou character, is a kid, because kids act like that.
Maria, the "unusually intelligent child", isn't necessarily more mature or sensible.
Kodaka is your dense harem lead archetype who's revealed to be simply faking being the archetype for personal reasons.
It's worth noting that Rika is the exception; her loveable sex maniac trait gives way to her more serious side. On the other hand, Kodaka is notably freaked out by it often enough. In a less serious series, no one would bat an eye at it.