Yeah its sort of ironic about how all three of the first films are about the characters meeting each other or becoming stars.
The Muppet Movie, I believe is an elaborate fabrication of how they might have meet, like MisterSuperStar said. Then GMC, is basically just them playing rolls, they are themselves but they are other people as well. Then in MTM, is basically just them wanting to become famous, which is basically just TMM with the muppets allready meeting and they just want to go to New York City to become Broadway stars.
Also, I love how a lot of fans believe that Kermit and Miss Piggy are married. They aren't, it was a musical, even though Rowlf was supposed to be the "Singing Preacher." I still dont believe they are married, just because Piggy hired a reall preacher. Remember, she also did this in the Marrisa Berenson episode of TMS[Season 3.]
I dont believe Kermit and Piggy are a couple, I think Miss Piggy believes and says they are a couple, but Kermit is just skeptical about the whole situation and just thinks that they are friends, Miss Piggy wants their friendship to be with well, benefits. Also, in MCC, they are 'married' with figs, or progs. BUT, that was just an adaption, not in the whole Muppet Universe, so yeah, I dont believe they are in a relationship what so ever. I think Miss Piggy believes they are, Kermit is just playing hard to get. He also may just be interested in Miss Mousey though, [ahem.]
Oh, and there might be skeptics that the relationship is just an act in front of cameras, I highly doubt it, because Kermit insults Piggy wayyyy to much more, he should give the Pig respect if they are having a secret behind the scenes relationship. But I do think the Frog needs a girl/pig/frog/whatever.
[geez that was way to intellectual for my muppety geekiness]