I was going to say that "This sounds like the pot calling the kettle black," but I suppose that's also now racist. However you choose to express it, this person is definitely a few cars short of a train.
Her cornbread ain't done in the middle.
She's a few fries short of a happy meal.
She ain't paddling with both oars.
Her elevator don't go all the way to the top.
She "just ain't right".
The lights are on but nobody's home.
She's couple eggs short of a dozen.
She is suffering from delusions of adequacy.
Her cheese done slipped off the cracker.
Calling her an idiot would be an insult to idiots.
The wheel is still turning but the hamster is dead.
She's like a pair of kid’s scissors, bright and colorful, but not too sharp.