u/AporiaParadox

Spider-Man. Spidey is teaching at Avengers Academy, and a student there called Jack (who is technically not a real student, long story) delivers an essay that is just a Spider-Man/Human Torch slashfic with suggestive drawings. It's apparently so bad it triggers his spider sense.

Ms. Marvel. Kamala Khan herself is a fanfic writer, frequently shown writing fanfics about Captain Marvel. But she gets outraged when others write fanfics about her that depict her in ways she doesn't like, such as one about her being in a love triangle with Miles Morales and Nova. As a bonus twist, it turns out said love triangle fic was written by Miles.

Adventure Time. The episode about Fiona and Cake is revealed to be just a Rule 63 fanfic written by Ice King that Jake and Finn are forced to listen to. We eventually get the Fiona and Cake spin-off where said fanfic becomes real.

South Park. Tweek and Craig were a relatively popular ship in the fandom, so South Park made an episode where the girls in town became obsessed with drawing Yaoi fanart of the 2 kids (and the creators actually used real fanart in the episode) for no real reason, to the point that everyone is convinced that Tweek and Craig are gay. Tweek and Craig eventually do become a real couple.

J. Jonah Jameson is willing to believe in Spider-Men and Green Goblins, but a giant purple alien who wants to devour planet Earth? Clearly a hoax.

Squirrel Girl goes on a blind date with a guy. He turns out to be a superhero truther convinced that all heroes and villains are fake.

Mr. Terrific refuses to believe in God, souls, or the afterlife in a world where these things are blatantly real and he frequently interacts with these things.

The Infinity Conez in Thor: Love and Thunder. The Infinity Gauntlet was used to genocide half the universe, using it as a prop for an ice cream stand would be pretty messed up for everyone, both those who mourned their loved ones and those who were dead for 5 years.

Many DC and Marvel stories have people dress up as the Joker and other villains for Halloween and costumed celebrations. That would be like people in the real world dressing up as real world mass murderers and terrorists.

Star Wars. The Empire has a very widespread propaganda network designed to control the narrative in the Galaxy, where the Empire are a noble and righteous institution that brings peace and order, and the Rebels are all terrorists. We see how effective it is in Andor, where for years they set up a narrative to turn the Galaxy against Ghorman so they won't care when the Empire carries out a genocide and strip-mines the planet. In another story we also see that the Minister of the Imperial Propaganda division be very annoyed that the Emperor and Darth Vader being so blatantly evil and making no effort in hiding it makes her job really hard.

Avatar: The Last Airbender. We see how the Fire Nation uses propaganda to control the public opinion of their citizens, especially in the Ember Island Players' episode where the heroic fire nation defeats the inept Team Avatar and the traitorous Zuko and Iroh.

Transformers IDW. After becoming the leader of Cybertron, Starscream tried to make a biopic movie where he was a good guy who really cared about Cybertronians and humans, and he turned on the Decepticons for going too far, notably blaming Megatron for shooting down Air Force One when he was the one who did it. In the end though, Thundercracker rewrites the script and makes a more warts and all biopic, Starscream then tries to have Thundercracker killed.