Uh, yeah, he's bonkers as portrayed in South Park, all right-- hootin' and hollerin', leaping around his mansion in what may as well have been a rotoscoped Daffy Duck routine. However: I don't know what Gibsonian antics Sullivan is thinking of, but I've seen no evidence that Mel deserves the treatment that South Park gave him.

The episode is all about how The Passion supposedly states in no uncertain terms that The Jews™ are collectively to blame for killing Jesus, which naturally inspires Cartman to don full Hitlerian regalia and begin leading marches against synagogues (until it's revealed to him that Mel Gibson is in fact kaka-cuckoo, upon which discovery he retires home in abashment). I guess Parker and Stone must have seen the movie, but it seems to me that they must have deliberately missed the point of it, because the South Park episode in question is founded on a straw-man argument and ultimately ends up being weak and confusing.

Besides which, there's the seemingly important argument that the narrative paints Jesus' death as predestined-- that the whole point of his birth and life as a human was to suffer and die for everybody else's sins. (Parker and Stone bring up this point, but don't bother addressing it.) Without that unjust death, that martyrdom, there would be no Christianity-- Jesus, divine or not, would have lived an obscure life of traveling ministry, evidently never to make an impact on theology through the ages. Which makes the question of "who killed Jesus?" rather moot, it seems to me; are the people who blame it on the Jews actually saying they'd prefer it if there had been no Crucifixion, and therefore no Christianity?