The Simpsons currently resides under the Disney corporate umbrella alongside Marvel, but Marvel characters have appeared on the show for years. Many fans wondered if Disney buying X-Men-owners Fox would change The Simpsons, but so far, it doesn't seem to have had any noticeable effect. To be fair, that's likely due to Disney not really caring what the show does, as long as it continues to make them boatloads of money. That always seemed to be Fox's philosophy, as The Simpsons never hesitated to bite that hand either.
To be sure, The Simpsons probably wouldn't get away with some of the jokes it does were it not such a long-running, honored institution of primetime TV. Having been running for over 30 consecutive years, The Simpsons has had occasion to make fun of just about everything, from corporations, to movies, to other TV shows, to video games, to politicians, musicians, and many others. The Simpsons has never really spared any group from its normally lighthearted jabs.
Over the years, The Simpsons has had lots of fun at the expense of both Marvel and DC, especially since superhero projects began to dominate overall pop culture in the early 2010s. This has extended to multiple Marvel Comics characters making cameos, whether as themselves or very thinly veiled knockoffs. For the sake of clarity and brevity, the following round-up won't include times Simpsons characters cosplayed as Marvel characters or characters that were referenced but didn't appear onscreen.
Stan Lee - Multiple Appearances
Prolific Marvel character creator, MCU cameo machine, and all-around comic book icon Stan Lee has appeared onscreen in four different Simpsons episodes. Lee first appeared in season 13's "I Am Furious (Yellow)", turning up at the comic book store to judge Bart's original comic creation. He later showed up in season 24's "Married to the Blob," officiating Comic Book Guy's wedding. Lee was also part of season 28's "The Caper Chase," appearing in an X-Men-themed couch gag. Finally, Lee pops up during season 29's "Springfield Splendor."
J. Jonah Jameson - "Moe'N'a Lisa," Season 18
In "Moe'N'a Lisa," Lisa turns Moe's notes into poetry, which gets published by Spider-Man's boss J. Jonah Jameson. He later requests both pictures of and poetry about Spider-Man, because this is Jameson after all. The best part is that Jameson is actually voiced by movie actor J.K. Simmons, and even drops a line from Spider-Man 3, which would come out not long after the episode aired.
Nick Fury - "Dark Knight Court," Season 24
Season 24's "Dark Knight Court" contained a sub-plot revolving around Mr. Burns' desire to be a superhero called Fruit Bat Man. As part of the story, Nick Fury appears, and while he's sadly not voiced by Samuel L. Jackson, he's clearly based on the MCU version. When Burns is captured, Fury assembles a group of elderly superheroes dubbed The Dependables, an obvious Avengers knockoff, as that movie had released the previous year. In an ending tag, Fury is recruited to join a team of the best team assemblers in the world.
Black Widow - "Married to the Blob," Season 25
Scarlett Johansson's Black Widow made an appearance in Simpsons too. There's no Simpsons character more connected to Marvel than the man known as Comic Book Guy, and in season 25's "Married to the Blob," Jeff Albertson ties the knot with his Japanese girlfriend Kumiko. During the episode, Comic Book Guy sings a song and is at one point joined by the recurring character of rival store owner Milo, and it's revealed that Jeff has a life-size cardboard cutout of Scarlett Johansson's Black Widow.
Badgerine - Season 25 & Season 29
While Badgerine is only seen in statue form, the creature is worth mentioning, both because he's a hilarious-looking parody of Wolverine, and because he appears in more than one episode. Badgerine is seen inside the comic book store in both "Married to the Blob," and "Springfield Splendor," which as mentioned previously, both included appearances by Marvel godfather and cameo king Stan Lee, and are both chock full of comic references.
Silver Surfer - "The Man Who Came to Be Dinner," Season 26
"The Man Who Came to Be Dinner" is one wild Simpsons episode, taking the show places it doesn't normally go outside of the annual Treehouse of Horror stories, and featuring aliens Kang and Kodos. Through Disneyland-related shenanigans, the family ends up blasted off into space, landing on the drooling monsters' home planet. As they travel, the ship passes by The Silver Surfer, complete with a female companion riding on his shoulders. The aliens later attempt to eat Homer, but the Simpsons escape. As one might imagine, this episode is considered non-canon but enjoyable for The Silver Surfer nonetheless.
Loki - "Bart's New Friend," Season 26
Season 26's "Bart's New Friend" saw a hypnotized Homer mentally revert to 10-years-old. By the end of the episode, the hypnotist named Sven Golly is liked inside a clear prison cell. He tricks Chief Wiggum into letting him escape, but then Loki randomly teleports in, shocking Springfield's most incompetent lawman. This appears to reference Loki in the prison cell during The Avengers.
Ben Parker - "3 Scenes Plus a Tag from a Marriage," Season 29
Spider-Man's deceased uncle Ben Parker makes one of the weirdest cameos in the history of The Simpsons, which is saying something. In "3 Scenes Plus a Tag from a Marriage," the family goes to see the latest Marvel epic, which includes a hilarious send-up of the MCU's trademark credits scenes. The best one sees Uncle Ben revealed to be both alive and evil, transforming into a giant demon, sitting on a throne of skulls, and dropping an excellent parody of his most famous Marvel line: "With great power, comes no responsibility." Ben tops that off with a maniacal laugh.
Thanos - "The Girl on the Bus," Season 30
Thanos the Mad Titan, perhaps now Marvel's most infamous villain, appeared as part of a genius couch gag in season 30's "The Girl on the Bus." The Simpsons episode released a few months before Avengers: Endgame, and sees Thanos insert Maggie's pacifier into the Infinity Gauntlet, then use it to snap the Simpsons to ash, except for Maggie. The two then prepare to watch TV.