Stan befriends a Mormon boy and his family, but Stan feels that the Mormon boy's beliefs are very odd.
"All About Mormons" contains examples of:
Anachronism Stew: During Joseph Smith's lifetime, the term "Native American" referred to white U.S. nativists, not American Indians.
Bait and Switch: During the narration of the Smith story, an upbeat tune plays in the background, with a choral "Dum, dum, dum, dum, dum" following the lyrical lines of the song that tell Smith's claims. When Lucy Harris appears in the sub-story and points out the flaws, the chorus changes to "Smart, smart, smart, smart, smart", and it becomes clear that the voices are actually singing "Dumb, dumb, dumb, dumb, dumb" in regards to Smith's stories and the townspeople believing them.
Decon-Recon Switch: Most of the episode mocks the story of Joseph Smith and Mormonism's founding (with the implication that Smith made it all up just to make money). However, the episode ends with Gary telling Stan that even if Smith did make it all up, Mormonism still promotes good values and his family is happy, loving and functional thanks to them following and believing in the teachings.
Only Sane Woman: In the story of Joseph Smith, Lucy Harris is the only one who realizes it's more than likely that Smith is making it all up. This is enforced when she asks questions about the Plot Holes, the background music to her says Smart to everyone else's Dumb.
Subverted with Stan. While Stan isn't incorrect to call the ideas of Mormons dumb, he comes across more as an intolerant know-it-all Jerkass to Gary, who just wanted to be his friend.
Phrase Catcher: Whenever Smith explains why he can't show proof of something, the person or people listening to him respond with an awestruck "wooow!"