Remember when people could just appreciate the Simpsons and not complain constantly because they didn't keep the same magic for 3 decades as if that was ever possible?
The Simpsons was one of the biggest and influential comedy sitcoms of the 90s Now in the 2020s I feel like it’s an underrated show for how a comedy sitcom can be written. It’s fucked up because since that time, new episodes are still coming on tv, quality be damned.
Honestly not a very Simpsons-y joke. It's funny but just really broad like it could be from any '90s sitcom. It makes sense that it's from pretty early on before the writers had nailed the show's tone and off-beat humor.
In sitcoms of the 1990's there was usually a jail scene, and someone would be playing a harmonica, which made the atmosphere seem dreary and hopeless, like how real jail is. This guy said he's basically only there to set the scene and make it feel like a jail. Homer asked him what he got arrested for, and he replied "atmosphere." Instead of being there for committing a crime, he's there only to make the scene seem like an authentic prison scene. That is the joke, because why would someone be in jail for such a ridiculous reason? It's silly. Hope that helped.