Almost every story starts with a mistake of some kind of mistake, whether it is a misunderstanding, a lie, or just a dumb decision. The last option is often the most common since human beings have a habit of doing dumb things in moments of passion, panic, or hurry.
It's a good thing, too, because without any of the following dumb decisions there wouldn't be any Star Wars movies to enjoy. When arguments erupt about Star Wars, it's a guarantee it won't be about how clueless the characters are since that's the main driving force behind the plot most of the time. For the most part, we'll be merciful with this lot, as we know they don't exactly have the world's best writers backing them up.
6 Luke Skywalker
Luke doesn't think it'll "run" if he takes the restraining bolt? It has wheels, doesn't it? This character has more of what people refer to as "emotional intelligence" and a closer look reveals that most of the Jedi Knights seem to have this trait.
Some of Luke's dumb decisions can be passed off as childish naivete in the first movie, but he never shakes this off. What did he think would happen in Return of the Jedi, that his father would betray his master, the person who had saved his life decades ago and had been his teacher and mentor ever since, just to save a son he barely knew? On the other hand, maybe that's not the best example.
Luckily for the screenwriter, Luke's powers of dumbness are that special type that saves him from peril as opposed to getting him into it. He was absent when the Imperial forces came to his house looking for that runaway droid, for example. Was it just dumb luck, or was it...the Force?
5 Count Dooku
Count Dooku appears in a variety of Star Wars media, including the prequels and just about anything involving the Clone Wars, and makes all kinds of dumb decisions in all of them. It's understandable at first that he leaves the Jedi Order because of corruption, but then he became a Sith, which is the pinnacle of corruption. Wait, what?
Dooku seems to be good at making nefarious plans but not very good at carrying them out and does nothing to hide his frequent absences or generally secretive behavior. Never was there a more obvious villain than Dooku, but thankfully the Jedi Order is also pretty clueless, and they miss him completely despite the fact that he's so dumb about hiding everything he does.
4 Yoda
Most Jedi make dumb decisions pretty much all the time, except for a precious few, and one of those would be Yoda. This is why Yoda's occasional and weird episodes feel so glaring and inconsistent. He makes bad choices every so often just to move the plot along or create superficial conflict where there normally wouldn't be any.
The scene when the audience is surprised by his identity is cute, and Frank Oz should have won an Oscar. It's mostly for the audience, however, because wouldn't it have been smarter for Yoda just to tell Luke who he was?
It gets worse. In The Empire Strikes Back, when Yoda first appears, he insists that Luke sacrifice his friends and not face Vader. Whether or not this is dumb might be a matter of opinion, but when compared to his total turnaround in the next movie, that Luke can't even be a proper Jedi without confronting Vader, it's just dumb.
3 Asajj Ventress
Asajj Ventress puts on a great show, but this very capable Sith gets in over her head too quickly, and her superiors eventually view her more as an expendable asset than a true master of the Force. Count Dooku was actually her master in part of the lore, and she definitely inherited some of his poor judgment.
Darth Sidious sends her to fight Anakin simply as a test for the Jedi, knowing that she'll fail. Either she knows it, too, and her pride makes her go ahead with it anyway or she's dumb enough to think she can survive. Then again, the Jedi themselves are renowned for their own poor decisions, and no other Jedi has a worse reputation for that than Anakin.
2 The Emperor
This character is both evil and dumb, and it's a hilarious combination that's at least fun to watch. There might be a method to this madness, and the Emporer seems to know he's wearing some sort of plot armor that keeps him from permanent death maybe he's just having some silly old fun.
There are plenty of obvious examples here throughout the movies, like taking on Anakin as an apprentice and toying with Luke until it was too late, along with everything else in between. Then there are the more obscure choices, like sending Asajj Ventress to die fighting Anakin. According to the lore, she didn't die but lived on, and eventually used her powers to help a number of Jedi such as Obi-Wan Kenobi and Ahsoka Tano, so nice blunder there.
1 Han Solo
This character is supposed to do dumb things, so maybe this is too easy. Han Solo is the lovable meathead that can't ignore his inner compassion, and this clashes with the more pressing need of paying off a ruthless Hutt gangster. This is partly why he's blinded by an old man's obviously hollow promise of wads of cash for a simple delivery job, with payment on delivery. Talk about the Worst Gangster Ever.
It's how he got into this mess in the first place, obviously, and nobody is surprised when he makes a few more dumb moves and gets in even deeper. This might have something to do with the princess he fell in love with, and people do all kinds of dumb things for love.