I think the vulgarity might just be what keeps it from being ableist. I'm learning in this thread that most commonly-used words for "unintelligent" have historically been used for actual diagnoses in the medical field for people with actual disabilities, which inexorably ties the word to the concept of being unintelligent by necessity, instead of by choice. So, something vulgar that would never be used by the medical field for a real disability can, at least in theory, be used to describe someone as being willfully ignorant without the baggage of a medically-oriented usage history.