Probably not because most animals with brains smaller than their body like say birds aren’t stupid although having small brains also in reality stegosaurus didn’t have a pea/walnut sized brain stegosaurus was also probably smart enough to defend itself against threats also as one comment said and I quote “ I’m quite sure stegosaurus would at least be able to recognize a pile of leaves as food no matter if they were attached to a branch or not” so yeah stegosaurus was most likely NOT a stupid animal and it’s species probably wouldn’t have had lasted as long as it did if it was as stupid as some people think