In all fairness, the guys of South Park have ripped on almost EVERY major religion (starting w/ the jewish faith, Stone being jewish himself). Jew jokes are probably the most common on the show. They've also ripped on atheists.
They've made fun of mormons in one SP episode too - though the episode closed w/ the mormon kid teaching a lesson to the kids.
Incidentally, in South Park, only mormons go to heaven because God has declared it was THE right religion. ;)
Those guys make fun of everything - including themselves. Which may or may not suit anyone at any given time.