Here's a few gewd ones:
Yo Mama so stupid, she stopped at a stop sign and waited for it to say go.
Yo Momma so fat, people jog around her for exercise.
Yo Mama so stupid, she jumped out the window and went up.
Yo Mama so stupid, she got locked in a grocery store and starved to death.
Yo Mama so stupid, that she tried to put M&M's in alphabetical order!
Yo Mama so stupid, she asked you "What is the number for 911"
Yo Mama so stupid, she took a ruler to bed to see how long she slept.
Yo Mama so stupid, she studied for a drug test.
Yo Mamma so ugly, she went to a haunted house and she came out with a job application.
Yo Mamma so ugly, people go as her for Halloween.