Bowser is dumb for various reasons:
1) being a game series aimed at children, most of the villains are usually the joke-type of villain that do evil things even if they theorically wouldn't benefit from them that much either because they need the hero to seem smarter than the villain or to allow them to be defeated, in this case it's both for Mario as Bowser having a huge "PRESS ME TO KILL BOWSER"-button/axe that destroys the bridge he's on and he doesn't even attempt to stand on the safe floor (or, you know, press the button himself as soon as Mario tried crossing the bridge?) clearly shows he's made to be dumb-is for the sake of him being the villain of a children's game
2) technically there are still smart villain who manage to get defeated even in media aimed at a younger audience, but in Mario the hero is an ex-plumber who's only abilities are an above-average agility (especially jumping), a lot of bravery and being great at fixing broken pipes (not that it'd matter as fixing the Mushroom Kingdom's sewers won't be something that'll help him rescue Peach), while he's still a pretty strong and prominent guy compared to many of the inhabitants of the Kingdom (who are usually either weak, evil or, if they are strong, are cowards like Toads)
So yeah, the series being aimed at children and the fact that Bowser is just a lighthearted joke-villain in a world of sunshine and rainbows is why he seems so dumb, he's supposed to be or the series would lose its child-like and happy-go-lucky feel, as having Bowser to either truly evil things or actually act smart and be a real threat to the Kingdom might make the game significantly darker in a way or another.