Wiktionary
nutty as a fruitcake
a. (context simile colloquial English) Behaving in an eccentric, foolish, or kooky manner; very nutty.
Usage examples of "nutty as a fruitcake".
She could be as nutty as a fruitcake as long as she did her job just as well tomorrow.
And the only reason he didn't was that if he did, his sister would have raised me, and she's as nutty as a fruitcake.