My definative answer to the posed question is: Yes, the term "Muppet" can be and has been used as an insult in the US, and well as other places around the world (I assume, being the UK members stated as such).
I'm sure it stems from the childish nature of the characters, but moreso puppets in general, being they are so often associated with children and things of an infantile nature (thus not fit for an "intelligent", "thinking kind" of person). So, in a way, I think we can kind of be happy that people call others "Muppets" rather than some other troupe of puppets, being it shows that the Muppets are the most popular and well known. I mean, you kind of have to be a bit of an idiot to use the word "Muppet" in a malicious manner, as we all know that a) the Muppets branch out so much father than Kermit, and b) Muppet comedy is SO much better than that of most humans.
So, yes it can be an insult, but I wouldn't really delve into it very deep.
-Matt