All of the four main characters of the Avantasia Protag AU series have their turns at being ditzy. Some of it is justified because of the time travel leading to unfamiliarity with modern things, but often they do wacky or stupid things with little excuse. The author describes them as "sharing a single brain cell which gets passed around between the four of them."
The Boy Who Cried Idiot: Martin is the eponymous idiot, doing things like assuming Lincoln is so-named because he's related to presidents, getting people's gender wrong, and forgetting things moments after doing them.
Cao Hong in Farce of the Three Kingdoms. The rare occasions when he actually does something sensible are considered remarkable.
Limefrost Spiral in Manehattan's Lone Guardian is described as being forgetful and not very bright outside her element. Her first appearance has her completely forgetting the time that she was supposed to meet Smooth Beat for a date, leaving him to wait at her door while she wasted time sunbathing.
Sayu: I guess. Why do we have to take a train or walk everywhere? You're way old enough to drive.
Light: It would take longer to drive than it does to walk. Have you ever seen rush hour?
Sayu: The movie?
In The Loud House fanfiction The Nightmare House, Leni's impostor is just as dumb as her and accidentally gives away the location of the real sisters.
Kayla in NoHoper, is an example of a ditz too dumb to take a hint and cannot fathom that Light is not in the least bit interested in her:
Light: I like... science.
Kayla: Oh? Really?
Light: Yes. And some silence.
Kayla: Is that a band?
Ash Ketchum of The Pokémon Squad, who often borders on Too Dumb to Live and/or Lethally Stupid at times. His stupidity and gluttony often kickstart an episode's main dilemma (such as "Unhappy Trails", where he mistakes RM's credit cards for Pokémon cards and trades them, which forces the entire squad to live in a trailer park. Ash burns down their trailer while imitating Beavis and Butt-Head.)
Spooky: Tombstones often creates problems for himself, whether it be by not thinking ahead, using his scythe as a universal tool, or just not comprehending how to tackle a situation.
This Bites!: With Vivi now having a permanent place on the crew due to her undeserved bounty, her tendency to act before remembering, considering or displaying all available information is shown in a lot more detail. An example is how she mistranslated the sign language of an ancient civilization of octopi and got everyone mad at the crew because she failed to read through the whole translation book properly and thus didn't notice the particular dialect for people with joints.
Unbreakable Red Silken Thread: Cody describes talking to Lindsay as "best to avoid allusion, irony, sarcasm, repartee, satire, and words longer than 'chicken'."
Deconstructed in Wishing Well with Rainbow Dash. He's described as "vapid and innocent", but this is due to essentially brain damage. He was fond of eating a fruit that was later found out to cause short-term memory loss. Rainbow Dash's memory issues and childish personality make him unpopular and awkward to be around.
: Italy sure seems like one. After all, he seems to be oblivious to a lot of things including Germany's and Japan's obvious feelings for him. Keyword: seemed.