It's really hard to do characters that are so close to the performer's real-life voice, like Fozzie or Kermit or Scooter. The extent of believable Kermit and Fozzie I can do is basically "aaaAaah", "wocka wocka", and "YAYYYYY!" (arm flails included). I can sort of fudge Scooter when I sing, but there's so much Richard in Scooter's voice that it's hard to master.
As I said before, Baby Kermit and Fozzie are so much easier.