I don't think it's a curse at all. It has to do with a performance in a play. The narrator's mother is the prompter of a play, and the narrator's grandmother is acting in the play. The prompter has to think clearly in order to give the actors the correct prompt. However, upon seeing Miss Frost, the narrator's mother was confused and stupefied. She couldn't think straight. She was so befuddled that she could have given the narrator's grandmother any ridiculous prompt besides "crap in a cat's litter box", such as: "prompted her mother to eat a cockroach"; "prompted her mother to set her hair on fire." Therefore, "crap in a cat's litter box" has no special significance. Lastly, you certainly look a lot like a young Christopher Lloyd.

It sounds like a joke referring to the context. Was there something to do with language confusion in the actual scene by any chance?

I've never seen it before, either. There may be some context I don't know, since I can't figure out why the author's mother would want her mother to defecate in a cat's liter box because of the presence of Miss Frost. Just the look on the younger mother's face would be enough, maybe?

Crap = to take a shit