It's not a bad idea, I just don't think it's a right tone for this film. Having her as a cameo wouldn't be too bad, but I could see things going really wrong, really fast if it isn't done right.
As long as it's tasteful and in the interrest of the 2 writers of this script, I'm all for it. But if it's stunt casting, I just don't see this being true at all.

The catch phrase "stunt casting" was made trendy when Neil Patrick Harris employed it to discourage sticking once popular media creation Britney Spears (not really an actor) on his and Segel's CBS show "How I Met Your Mother" for sweeps ratings. To associate that sort of terminology with Academy Award winning actor and critically acclaimed movie star like Jolie is preposterous.

I use The Muppet Movie as a guide map. It included classic talents like Edgar Bergen, Milton Berle, Bob Hope and Orson Welles, current 70s icons like Mel Brooks, Dom DeLuise, Madeline Kahn, Carol Kane, Cloris Leachman and Telly Savalas, along with the very popular movie stars Steve Martin and Richard Pryor among other stars of varying degrees.
I think some folk around here are over-thinking things. It's a fan forum. We do that. But an Angelina Jolie cameo is - a very good thing. The Muppets have always been about cameos and stars. Segel mentioned his intention was to go back to the trilogy and the show and that appears to be exactly what he's doing. Maybe some fans have other ideas, but the writer/director of this project has stayed true to his mission statement thus far - and with passion!

I hope we get a lot of movie star cameos. I also hope they stay merely cameos (and I think they will).