The origin of the word is not ableist.
This is precisely not about etymology. Modern linguistics is descriptive, not prescriptive (as is obvious to anyone familiar with dialectics and materialism). Words like "dumb," "slow," and "edit: the r-word" that have origins in explicitly ableist use are not worse than "stupid" in the way I'm criticizing them. Especially "dumb." It's synonymous with "stupid" and most of the time not used to mean deaf. So I'm not saying don't say insert common phrases in English because it's insert a bigotry endemic in Anglophone culture. Although, that is a valid concern on it's own and I don't care if someone doesn't say and/or don't want others to say "dumb" and "rule of thumb" for that reason.
TLDR: problematic etymology is a valid concern but lack of it doesn't prove the word doesn't cause harm. Usage in real world is more important.
Anyone can say or do something stupid and they aren't being compared to anyone but themselves when they do.
It's the judgement of the speaker to say "stupid." Or funny, or annoying, or valid, invalid, offensive, etc. While you can say "I expected more of you so I call you stupid now," that's it. You expected.
It's a measure of intelligence that you are imposing on others. I mean...what if I just called you stupid for disagreeing with me or not understanding everything I think or believe from the few sentences I presented. More than unfair, it's bigoted. I'm relying on our culture's shared idea of what intelligence means and what it means to have little of it, to insult you. I'm not saying the insulting is the problem. You can say "fuck you go to hell." None of that is ableist. Picking the person's perceived skin color to insult them is racist. Picking their perceived gender is sexist. And picking the perceived intelligence is ableist.
I don't think there's such thing as "but they are stupid" or "it was dumb." That's just our chauvinism. People, their ideas and actions can... lack foresight, care, attention, kindness, strength or bravery, and so on. But when you say it lacks intelligence. I think it's...well I'm going in circles.
TLDR: You're invoking the ableist world we live in when you say "stupid" to give yourself pover someone that you're saying it to. Same idea as how "cracker" is just a joke "slur" because we don't live in a world where whites are oppressed as a race.