From the oldies but goodies list:
Yo mama so ugly, she has to tie a porkchop round her neck to get the dog to play with her.
Yo mama so stupid she went to a movie and saw "Under 17 not allowed" so the bitch went home and got 16 of her friends.
Yo mama so fat, if she sat on a quarter, a buger'd come flying out of George Washington's nose.
Yo mama so stupid, she wondering why Elvis don't tour no more.
I made that last one up just now. Can you tell? Yah, I thought so.
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I ate a hooker half a bottle of knife.