Looking up words is not stupid. Misusing words because you assume you know what they mean, that is stupid.
But if you're sure you know what it means (albeit incorrectly), because that's what you've picked up or been taught and no-one has ever corrected or queried you, where would the impetus to look it up in a dictionary come from?
You might have a case for "deliberately misusing words [...] is stupid" but there's a long comedic and literary tradition there...
I also think there is a shared sense of how fancy, big or difficult a word or phrase is considered. Look up the fancier words more often!
Another possibility of course is wanting to correct somebody else, but double checking the definition just to be sure before you do!
Misusing words may or may not be stupid—without more information, including some nebulous stuff about intent and interpretation, I have no way of saying for sure in a given encounter.
What I would say instead is that many people seem to see “misuses words because [cocky/pretentious/know-it-all/careless]” as a heuristic for stupid. It isn’t a great heuristic, but knowing that people use it to rank you and choosing speech accordingly…isn’t stupid.
That's for the ML algorithms to decide.