I’ve seen the play (and the cartoon). The South Park defense relies on the notion that a work can say something offensive as long as it’s being ironic. This is different from the assertion that it’s okay for a character to express an offensive view as long as the work itself doesn’t defend that view. According to the character defense, the fact that the white missionaries have stereotypical views of Africans doesn’t necessarily mean that The Book of Mormon itself does. But in this case, the play is even more prejudiced than its brainwashed Mormons. The white characters at least question their assumptions, which is more than you can say for Trey Parker and Matt Stone.
The irony defense maintains that the work is talking ABOUT an outrageous perspective and we’re all too cool to be outraged by a work of fiction. An ironist defender might claim that The Book of Mormon DEPICTS racist attitudes but rather than endorsing these attitudes, it subverts them. “We’re all above that.” Or so we want to think.
While that defense / interpretation may be true for some works, I don’t think it validates the racist subtext (or main text) of The Book of Mormon. Irony in this case simply makes it easy for a white audience to ignore the fact that the white characters in The Book of Mormon are depicted as complex, conflicted individuals, while the Ugandans are shallow, ignorant simpletons. Rather than shocking the audience, as Ibsen maintained that drama should do, The Book of Mormon reassures its viewers — with a wink and a nudge — that everything is okay, as long as we can have a good laugh.
The fact that racism is invisible to people who are acculturated with it doesn’t mean that it doesn’t exist. As Martin Luther King Jr. pointed out, the “moderate” white Episcopal bishops, who claimed to be unprejudiced while arguing that King shouldn’t proceed too fast in his quest for equal justice because King’s non-violent protests could incite violence, were far more dangerous than the blatant racists like Bull Connor.