In the pages of Marvel Comics stories, it's not uncommon for secret cameos or easter eggs to be hidden within. In one of Marvel's biggest events ever, Eric Cartman from South Park made a surprise blink-and-you'll-miss-it appearance as a Skrull in disguise.
South Park is no stranger to the superhero genre, as they've frequently parodied Marvel Cinematic Universe films and other comic properties in their show. The video game South Park: The Fractured but Whole and the episodes the 2017 RPG was based on was a heavy satire of Marvel and DC Comics films. But, long before South Park parodied the MCU, Cartman made a secret cameo as a Skrull in the Secret Invasion event.
Secret Invasion (2008) by Brian Michael Bendis, Leinil Francis Yu, Mark Morales, and Laura Martin featured shapeshifting Skrulls invading Earth and hiding in plain sight as everyone from superheroes, celebrities, to regular folks. The entire hook of the event was that literally anybody could be a Skrull in disguise and one scene showcased that concept, as in the series fifth issues had the Skrulls revealed themselves to the general public. The Skrulls took many forms of famous people including Steve Jobs, Tom Cruise, Paris Hilton, Stephen Colbert, John McCain, and even Eric from South Park.
If you look on the left, you can see Cartman slightly obscured. Bendis, Yu, and/or Morales managed to sneak in the South Park character in a page that was so busy, it was easy to get him in there - but one that makes sense in the comic. Skrulls copied all kinds of people on Earth, including Oprah Winfrey, Barack Obama, the Pope, and Kim Jong-Il, and even managed to take over a cartoon character. It's a silly reference, but one that confirms even in the Marvel Universe, Cartman exists.
Does Cartman's appearance in the Marvel Comics Universe mean that other popular South Park characters also exist? Does Kenny exist? Randy Marsh? Mr. Hankey, the Christmas poo? Cartman's cameo means the answer is likely yes, even if they never popped up in subsequent issues. Even Marvel superheroes need entertainment and the South Park show is certainly worth watching. With the South Park crew existing in the pages of Marvel Comics, it would be hilarious to see Kenny take on someone like Thanos and die, followed by a classic "oh my God! They killed Kenny!" We can only wish.