The jurisprudence on depictions of Muhammad, et al differ upon scholar; the general consensus is that depictions of holy figures lead to adoration, reverence, and ultimately idolatry, the very principle which Muhammad denounced. Indeed, there have been many ways to swerve past this; Persian illustrators depicted Muhammad and Co. either with metaphysical parchment shielding the face or silhouetted with smokeless, ethereal flames.
Again, the above was in a positive light; the South Park thing has the intent of humor, and is barely acerbic. Muhammad is not even visually shown; only described. That is far less of an offense, and goes nowhere towards the brink of blasphemy. If one wishes to see a depiction of Muhammad that would most certainly earn the ire of many Muslims (and, possibly conscious Westerners), one need only read Dante's Inferno; even a picture would suffice: