Your mother is so dumb, when the cops knocked, she told the dungeon master to flush her Bag of Holding.
Your mother is so confused by Tolkien, she thought Arwen was Eowyn and Saruman was Sauron.
Your mother is so dumb, she blocked Cthulhu's number on her cell so he can't Call.
Your mother tried to slash the eye-stalks off the attorney general of the United States because she thought his name was Eric Beholder.
Your mother is so crazy, she tried to milk Shub-Niggurath, the Black Goat with the Thousand Young.
Your mother is so crazy, her SAN was reduced to zero.
Your mother is so dumb, she thinks Chimera is Russell Simmons' ex-wife.Your mother is so dumb, when she rolled D12, Eminem lost half his posse.
Your mother is so dumb, she thinks a d20 is a dodecahedron.
Your mother is so old, she paints Napoleonic war miniatures with the exact uniform colors and details she saw them in.
Your mother is so fat, her intestinal parasite is a purple worm.
Your mother is so nasty, she uses green slime to exfoliate
.Your mother is so fat and hungry, the green slime that's eating her can't catch up.
Your mother smokes so much weed, and that's why they call it White Plume Mountain.
Your mother is so forgetful, that's why they're called the Lost
Caverns of Tsojcanth.
Your mother is so ugly, I lost 2d4 SAN the first time I saw her.
Your mother is so ugly, she'd make a basilisk wince.
Your mother is so nasty, her picture and stat block were withheld from the First Edition Fiend Folio
in favor of something more pleasant (see Penanggalan
Your mother is so ugly, Erol Otus doesn't have the stomach to draw her.
Your mother is so dumb, she's the one who suggested using THAC0 in Second Edition.*
Your mother is so dumb, she made characters named Edward and Bella for a Twilight 2000
Your mother's marketing skills are so weak that GLEEMAX!!Your mother is so dumb, she invested in Palladium cause she heard Rifts was being developed for N-Gage.
[Switching to tv and movie geekery below...]
Your mother is so dumb, she makes movies where people throw their swords like spears.
Your mother is so dumb, she thinks the tv show Firefly
was based on the novel by Piers Anthony.
Your mother is such a Trekkie, Neelix.
Your mother is so ignorant, she thinks "T'Pau" is the onomatopoetic name of the last cleaning product hawked by Billy Mays.
Your mother is so dumb, she thought "Seven of Nine" was a cute kid assimilated by the Borg who was brought in to perk up Married with Children
after it jumped the shark.
Your mother is so dumb, she thinks Doctor Who's on First.
Bonus joke attempts:
Q: What do you call Firefly fans who are also Nazis?
A: Brownshirt Browncoats.
Q: What would you call a sidekick of Captain Kangaroo if he were a Nazi and a fan of Firefly?
A: Mr. Brownshirt Browncoat Green Jeans.
* Note that I never played Second Edition and have no personal opinion of THAC0, but heard so many complaints about it, it seems almost as big a pet peeve to D&D fans as people throwing swords in movies.