Baccano!: Isaac and Miria, the Outlaw Couple pair, are twice as flakey. For instance, they think the fact that no one has ever found gold in a given hill is a perfect reason to dig for gold there, and that to perform a "train robbery", you take a train somewhere, rob someone, and then use the train to escape.
Akihisa Yoshi from Baka and Test: Summon the Beasts is by far the stupidest person in the school. He divides pot noodle in half continually thinking that that way he can get infinite meals out of one serving. He even concludes that discovering this makes him a genius. Most geniuses don't spend their food money on porn in the first place. He explains that this why he is eating 1/67th of a pot noodle. Nobody is even surprised that he gets the math wrong there. When Hazuki Shimada turns up and calls asks for the big idiot Akihisa Yuuji Sakamoto says he's surprised that everyone in Japan knows of his stupidity. Akihisa insists it's not all of Japan, just the local area.
Bazil Broketail: Vlok is not the brightest bulb among the dragons of 109th. Still bright enough, though, to be a competent swordfighter and a soldier.
In Buddenbrooks, Tony is impressed by smart people (like Morten Schwarzkopf — a doctor-to-be with whom she falls in love, but can't marry him because of the Grünlich thing), but is neither book smart nor street smart herself, and calls herself "a silly goose" sometimes. Her poor relative Klothilde is even more so.
Dear Dumb Diary — Emmily. [Yes, she really spells it with 2 Ms. It's because it reminds her of candy that way — not M&Ms (those are Ws and 3s) but because "Mm" is the sound she makes when she eats candy.]
You remember Emmily — she was very sweet and we all loved her, but she was not the sharpest knife in the drawer. Emmily wasn't even the sharpest spoon in the drawer. Most of the time, Emmily wasn't even in the drawer at all. She was lost somewhere in the bottom of the dishwasher.
Diary of a Wimpy Kid: Creighton the Cretin (Greg's comic strip character in the first book) is this in spades. He's tricked into thinking his name is "Stewart Pid" (and then says his name is "Stew Pid"), mistakes a brick for a box, and asks a doctor to give him a new butt because his old one has a crack in it.
Razza from Don't Call Me Ishmael!. His grades are terrible and he isn't very intelligent, but he is very sociable and fun-loving.
Sometimes, Razza wasn't quite on the same page as other people — often, he wasn't even in the same library.
Maeve does her level best to be perceived as a narcissistic, hedonistic political non-entity. While there is some truth to the first two, Maeve is far more savvy and in control than her public persona indicates. When Billy tells her no-one buys the act, she pouts, but immediately becomes all business and casually provides important information the heroes have been busting their chops to get a hold of. Underestimate the queens of Winter at your own peril.
Abby of the Ordo Lebes comes across as scatterbrained through no fault of her own. Abby is actually pretty intelligent and perceptive, but she is also a very limited-ability precognitive with very little control of her ability. The reason she spaces out and comes up with non-sequiturs sometimes is that she sees the present and about a minute into the future simultaneously and occasionally loses track of which is which.
Lina the maid from Astrid Lindgren's Emil i Lönneberga. When the parish priest asks her who the first humans were, she answers "Thor and Freya". That doesn't make sense even in Norse Mythology!
Homura Hinooka from Fire Girl is a downplayed example. Her ditziness mostly stems from her status as being Book Dumb and being fairly incompetent in knowledge regarding "complicated" basics in life (she doesn't even know how to use the internet for starters) but she is actually fairly self-aware and perceptive otherwise.
The Girl Who Drank the Moon has Fyrian: woefully uneducated, unable to observe or retain information, and believes everything he is told, but through this becomes an enthusiastic, optimistic, and loving family member.
Journey to Chaos: Ponix Enaz is constantly forgetting to fully shapeshift back into his elven form and thus leaving strange body parts lying out, providing Too Much Information, or being so absorbed by puzzles that he neglects what's around him.
Jack Pumpkinhead from the Land of Oz series is always described as unintelligent. Some of the words used to describe him are "stupid", "dim", "innocent", "simpleton", "not known for his intelligence", etc. He might also have a bit of a neurological disorder, as he fails to pick up on vocal cues and sometimes takes sentences to mean the opposite of what they mean (he once called a ferryman 'nice' after he refused to let Pumpkinhead cross a river on account of not having any money).
Miss Marple: Lettice Protheroe from The Murder at the Vicarage is vague about everything, and frequently confused about what time and even what day it is. Several characters express the opinion that it is a form of Obfuscating Stupidity.
In the Modesty Blaise novel The Night of Morningstar, Earl St. Maur's wife Victoria is an airhead from a noble family who is incapable of maintaining a train of thought on any subject that doesn't involve horse-riding or sex.
Mother Goose: Simple Simon was one, "simple" being a common term for "stupid" when the poem was written. After his meeting with the pieman, he tries to fish for a whale in a bucket of water and then tried to find plums on a thistle bush.
Ratburger has Sheila, who is so dumb she thinks a drawn-on mustache is real and it's not even well-drawn.
Louie from Rune Soldier Louie has a complete disregard for his own or anyone else's safety: "When it comes to the odds, I have done worse than 20 percent!"
In The Shattered Kingdoms, Kira pretends to be like this at the Norlander imperial court, being flighty, talkative, politically naïve, and superficial. Some people see through it, but as she notes, it's pretty difficult to prove she's smarter than she acts, so it's still a good barrier against people trying to get anything from her.
Volatilus, the sweet but not too bright little dispatch dragon from the Temeraire books. The little guy can't even pronounce the titular dragon's name — the best he can do is "Temrer".
Temeraire: And where, pray, do you come from? Volly: I was hatched! From an egg!
Gourry Gabriev is nicknamed "jellyfish brain" because he lacks so much common knowledge about the world and is generally clueless in any circumstance.
Naga the Serpent, Lina's (self-proclaimed) greatest rival, gets into a lot of trouble with her lack of sense. In Naga's defense, Word of God implies she is mentally traumatized, so it's less that she's stupid and more that she's genuinely mentally unwell. Although it's probably fair to say she wasn't sensible, to begin with, given the strong hints she's related to Amelia and Prince Philionel, who are... definitely flaky.
Tawneee (yes, three 'e's), a minor character from Thud!, a pole dancer. A short, somewhat paraphrased, description: "It's as if the gods said, 'Sorry, kid, you're going to be thicker than a tub of lard, but it'll be okay — because you'll be very beautiful'." She thinks people come to see her because she's a good dancer and knows some "tricky steps".
Till We Have Faces: Redival is unable to focus on the Fox's classes and at times misunderstands his teachings. She ends up lacking the foresight to see her gossip and slander against her sister will do great harm to her family.
In Warrior Cats, Fuzz, the kittypet that Barley meets in Secrets of the Clans, is a cheerful ditz who is so literal-minded that he thinks Barley's name is "Erbarley", and upon being corrected, then thinks it's "Justbarley".
Who Wet My Pants: Reuben doesn't notice when he wets his pants and when it's pointed out to him, he thinks first that someone else did it and then that they sprang a leak.
Wolf Hall portrays Thomas Cromwell's son Gregory as a bit dim. While Cromwell is a highly savvy polyglot, Gregory is a poor scholar, guileless, and flighty; at one point Cromwell admits to a friend that neither Cambridge nor Gregory have anything for each other. Gregory does wise up as time passes and his father finds more effective mentors for him, but Cromwell actually likes that his son is so unlike himself even if it makes it difficult to relate — he might not be the brightest but he's a model gentleman. More importantly, having a genial ditz for a son is always leagues better than getting Granddad's drunken, abusive jerkassery repeated in the family tree...
