It's not ableist to insult someone's intelligence. Sure the terms dumb is ableist, insulting someone with such a low intelligence that they actively would be classified as having an intellectual disability would be ableist, or making fun of learning disabilities too. But making fun of a non disabled person who is just unintellegent wouldn't be ableist. It would be mean spirited and possibly classist (if they're just uninformed due to lack of resources to educate) but not ableist.
even if you use words/phrases/insults that are offensive to minorities towards people who aren’t minorities, they are still offensive to minorities. i get where you’re coming from, anon, but the word “retarded” doesn’t stop hurting me and other autistic people just because somebody says it to someone who isn’t autistic, and making fun of somebody’s intelligence might not seem ableist at first, but you’re literally being like “HA, YOU’RE UNINTELLIGENT, JUST LIKE SOCIETY CONSIDERS PEOPLE OF CERTAIN CLASSES AND DISABILITIES”
friendly reminder that words like “idiot” and “moron” literally originated in modern language in reference to intellectual disabilities, just like “retarded”.