It's true that Jim never wanted the Muppets to be pigeon-holed as kiddy entertainment, nor did he want to be stigmatized as a children's entertainer either; at the same time, however, while the Muppets have always indulged in a little adult humor, it was always sophisticated and tongue-in-cheek to the point that it wasn't entirely inappropriate for kids either - like Rocky and Bullwinkle, for example.
Adult cartoons, especially Seth MacFarlane shows, are far more blatant about it because they always go for shock value to see just how far they can push the envelope in getting away with anything.
Ironically, Caroll Spinney is a big fan of SOUTH PARK.
Sorry, but I really hate South Park. I'm not too crazy about Family Guy, either. When it comes to adult humor, I prefer it when it's subtle, such as The Muppets, and Rocky and Bullwinkle. Matter of fact, when I saw reruns of R&B as a kid, a lot of stuff went over my head. Watching it nowadays, I get a lot more of the jokes I didn't used to.
The trouble with shows like South Park and Family Guy is that they never come up for air. It's just continuous dark humor. I mean, it's like this: "Ok, I get it! The world is a horrible place, nothing is sacred, everyone is despicable, we're all doomed! I get it, South Park and Family Guy! You've made your point! Can we ever move on?"
I'll give the Simpsons credit for coming up for air from time to time, but again, even then, their humor and the Muppets' humor are significantly different, so it was a mistake to bring in Simpsons writers for VMX to try to give the Muppets more Simpsons-esque humor.