There are a whole lot of candidates for the Darwin award for the stupidest movie villain these days. Sometimes it's because the movie doesn't work unless the villain is dumber than a bag full of hammers, sometimes it's the consequence of a plot with more holes than a moth-eaten fishing net.
We have elected to make two small rules for ourselves in compiling this list: we've only selected one movie from the massive Bond canon, even though the appalling stupidity of Bond villains is so well known that it's practically a cliché.
We've also disregarded almost all of Hollywood's output in the form of childrens' movies or comedy, purely because so many of these films rely on having deliberately stupid or incompetent villains for their equally challenged heroic counterparts to defeat, or outlandishly silly plots to drive the narrative. In those contexts, the villains aren't the dumb ones, as the entire engine of the film is one of enormous foolishness, craziness, and ineptitude.
After all, who could seriously expect anyone but Jacobim Mugatu to stand as the antagonist in Zoolander, or Mr. Lisp and Alexander Minion in Spy Kids?