Sure. Politics is surely filled with examples like this. I've often thought that movie villains are absurdly evil. I think most real villains in the world don't cackle with fiendish laughter as they gloat over how they're going to destroy all that is good and right and true. More often they say that they have to implement this program for the good of the nation or of the world, and these narrow-minded people who are standing in their way just have to be dealt with, for the greater good. Or, I just saw an interview on TV last night with some convicted con artist who talked about how this was just the only way she knew to survive, and she was sorry if others got hurt but what was she supposed to do? Etc. Oh, not that there aren't people out there who will kill someone just for kicks, but they're not the most dangerous villains. It's the well-meaning people who really believe in freedom of speech, but that group over there is just too extreme and they are really a special case, why, if we don't shut them up, etc.
For example, Neville Chamberlain is widely regarded as having blundered badly when he gave in to Hitler's demands rather than standing up to him. Many speculate that if Chamberlain had followed Churchill's advice and taken a hard line with Hitler, World War 2 could have been prevented. I'm sure we could debate the what-if's endlessly, but if that's true, note that no one is saying that Chamberlain was an evil man. Quite the contrary. He was a nice man in a situation where being nice just wasn't going to work.
I started to write up another example and I can easily think of many more, but you get the idea.
Note that for a politician to cause massive harm, he can't be a total bumbling idiot. He has to be pretty capable to get elected or to organize a coup or however he comes to power. But I think we've seen historically that the skills required to get elected or to lead an army are not necessarily the same skills that make one able to run a national economy, etc.
By the way, I deliberately avoided using a contemporary example above. If I said that Obama is a well-meaning fool or that Bush was a well-meaning fool or Samaris or Merkel or some other current politician, half the people here would agree with me and half would say that was outrageous. I can point to politicians in the last few decades with completely opposite policies, both of whom had wide popular support. So what makes a stupid policy is not obvious to everyone.
I've been talking about the harm of stupidity on a national level, but surely people do themselves and others plenty of harm on a more personal level. I suspect that most of the problems that people have in life are caused, not because they are the victims of some evil person, but because they make personal decisions that are obviously bad ideas. Not to say there aren't people out there whose lives have been ruined by a violent criminal or an oppressive regime, but I think many more people suffer because they drink too much or use dangerous drugs, or they can't hold a job because they don't bother showing up for work, or they marry someone that they know is an abusive jerk, or they ruin a perfectly good marriage by having a tawdry affair, etc, etc.
Again, though, I wouldn't make such a "stupid villain" just incomprehensibly stupid. I'd make him follow some impulse that, even if the reader should instantly see that it's stupid, we can see how a normal human being would do that. Like, the person doesn't quit his job for no reason at all, but he has a series of escalating conflicts with the boss until he quits. He doesn't just decide one day to become a drug addict, but he has some severe personal problem that leads him to try drugs just once to escape, and then the next time a lesser problem leads him to turn to drugs, etc, until he's using drugs to escape from life in general. Basic lust leads people to do all sorts of things that are objectively stupid, and I think we all understand that immediately.
