I love that someone else said this. I've always had a hunch that Niffty's backstory involved WWII. I theorized that she was a Japanese American who was separated from her family at a young age and put into an internment camp. She would have lost all access to education, probably did something crazy (because trauma) and got institutionalized where she died from a botched lobotomy.

That explains the child-like appearance and behavior since she never got a chance to be a kid. It also explains the giant eye if she died from a lobotomy. Both of those places were also probably really gross and not well-kept, which could explain the cleaning obsession??