Okay, let's settle this
Look above, at the section titled "Peter and the 4th Grade."  The response to that question is, and I quote, "No because it is just a joke. Using throwaway comments or jokes as a source of information is specifically mentioned as something to avoid in Manual of Style (writing about fiction)."

The gag about creationists being stupider than retards is only a throwaway gag in the episode Petarded, and is never even spoken orally, only written on a cardboard sign.  Therefore, it doesn't belong here.!  If it were a recurring gag, like that of Ernie the Giant Chicken, then it would belong, but it only makes a cameo appearance in one episode, and therefore is not notable enough to be included anywhere but the cultural references section of the Petarded article.