i made this one up a long time ago, and its proly not original, but: yo mama so fat, she went swimming in the ocean and the whales started singing "WE ARE FAMILY GET YOURE FAT *** DOWN HERE AND SEE"
Yo mama so ugly, she went to register for an ugly contest but the lady behind the register said, "Sorry, no professionals." Yo mama so stupid, she took a ruler to bed to measure how LONG she slept.