Here in Florida, we say crazier than an outhouse rat.
I've heard that one with a less polite word than "outhouse".
I'm reminded of something that Buffy said about Faith on Buffy the Vampire Slayer
: "The girl's not playing with a full deck, Giles. She has almost no deck. She has a three."
Welcome, Monsoon! I don't suppose you got your name from Sheila Chandra's wonderful band from the 1980s?