You’re as useful as a football bat.
You’re as useful as a soup sandwich.
Your momma is so dumb, on the way to the airport, she saw a sign that said “Airport Left.” She turned around and drove home.
There’s a good one near the beginning of The Tempest, but I think it could use a little updating. How about this:
You should take up skydiving because everyone knows the only place you’re going to die is in the electric chair.
It’d have to be changed some, but I still giggle when I think about it and it’s old.
From Welcome Back Kotter:
Horshack: “Hey, Mr. Woodman! Why don’t you put your false teeth in backwards… and bite yourself to death!”
Also, “Yo mamma’s breath is so stank, when she opens her mouth to yawn, her teeth duck!”