Maxwell Atoms, the creator of The Grim Adventures of Billy and Mandy, says that he's unlikely to work with Adult Swim because the network's current president once blamed him for "ruining Cartoon Network's brand", firing him shortly afterward.
Atoms, who regularly answers fan questions on his Tumblr account, revealed this story in response to an anonymous question asking if he ever considered making any shows for Adult Swim. "I have a couple of ideas that I think would kill on Adult Swim," Atoms wrote. "But since the current president is the guy who gave me a public dressing-down for 'ruining Cartoon Network's brand' with my 'hateful fart cartoon' before promptly firing me… such a collaboration seems incredibly unlikely."
The current president of Adult Swim is Michael Ouweleen, creator of Harvey Birdman: Attorney at Law, who has worked for Cartoon Network in varying capacities since its creation in 1992. Ouweleen was the Senior Vice President of Development and Programming at Cartoon Network from 2006-2008. The Grim Adventures of Billy and Mandy ended in 2007; a 2008 TV movie, Underfist: Halloween Bash, was meant as the pilot to a spin-off series but said series was ultimately canceled. It seems Underfist's cancelation coincided with Atoms' rather intense-sounding firing.
It's curious that Ouweleen would blame Billy and Mandy for "ruining Cartoon Network's brand." The cartoon about two kids and their best bud the Grim Reaper was one of the network's more popular shows in the 2000s, and it launched the careers of artists like C.H. Greenblatt and Thurop Van Orman, the creators of Chowder and The Marvelous Misadventures of Flapjack, respectively. You could single it out for being darker and arguably more mean-spirited than most of Cartoon Network's other original programming, but the "hateful fart cartoon" descriptor is an odd one.
Since leaving Cartoon Network, Atoms has worked at Disney on Fish Hooks and at Warner Bros. Animation on Bunnicula, Teen Titans Go! (which does air on Cartoon Network) and two Scooby-Doo movies. He attempted to produce an independent post-apocalyptic adult puppet show called Dead Meat via Kickstarter, but the production fell apart due to issues with expenses and contracts. He's been on good enough terms with the current Cartoon Network management (outside of Adult Swim) to attempt to pitch a Billy and Mandy sequel movie this past summer, though the pitch was ultimately rejected due to kids today not being familiar enough with the series.
Source: Tumblr