(as) funny as a crutch
Not funny or amusing at all. Yeah, cancer is about as funny as a crutch, huh?
*funny as a crutch
not funny at all. (Sarcastic. *Also: as ~.) Your trick is about as funny as a crutch. Nobody thought it was funny. The lame joke Ron told was as funny as a crutch, and we all yawned.