Humanoid Muppets (Statler & Waldorf etc) are very similar to humans, but are different. The whole song "Man or Muppet" in The Muppets (2011) makes this very clear, with Walter and Gary debating which they are
"Am I a man or am I a muppet?If I'm a muppet oh I'm a very manly muppet.Am I a muppet or am I a man?If I'm a man that makes me a muppet of a man"
Both the lyrics and the accompanying video (with human Walter and muppet Gary) suggest it may be possible to cross over between them, but at the end they both decide to be what we have always seen them as (Walter a Muppet and Gary a Man) which means the song was more about living up to what they are than really offering the possibility of change.