From Post-Punk:
"South Park creators Matt Stone and Trey Parker are admittedly huge Cure fans who got Robert Smith himself to voice his cameo on the show.
Smith later told SPIN that his nieces and nephews were huge fans of the show and stated that:
“of all the things I’ve done, all the people I’ve met, it all counts for nothing compared to being on South Park.”
Read the full article at Post-Punk.