I just happen to be reading Rebecca, which does indeed use "idiot" as a descriptor for someone who has some sort of developmental or mental disability (it's not at all specific).

In those cases, maybe the word "fool" (caveat: I've known people who would freak out about having that word used on them because of its connotations; however, I don't think it has ableist connotations--more "you should've known better" connotations, which seem to be what you're looking for)?