Nick is actually in the above average range of intelligence, but he has schizotypial personality disorder, so he lives in his own world. His actions make sense to him, but only to him. To those of us in the real world he's crazy, but he's not stupid in the sense of low IQ. Since he doesn't follow the dictates of normal society, that's why he bragged about molesting his relative. But he also had enough sense to deny the accusations and try to prove his innocence. Yes, it totally backfired on him, but in his mind it made sense.
Striker I think is legit brain damaged, and probably wasn't that bright to start with. He might actually be clinically mentally disabled, as in an IQ lower than 70. He clearly can't take care of himself, and will probably require assisted care for the rest of his life, however short that might be.
Shaner I think has average intelligence, who received poor instruction in school but has untreated autism which lead him to react the way he does, but he's also a sociopath, a low-functioning one, but that explains why he's such a massive asshole.
TJ Church is the Dunning-Krueger effect personified. He honestly doesn't know he's an idiot.