muppet (ˈmʌpɪt) — n slang a stupid person
Is that also the case in American slang?
In American English, 'muppet', capitalized or not, has no meaning other than being characterized by the Jim Henson branded characters. The idea of it being a 'stupid person' is unknown. Some of the Muppet characters are slow, others are bright, others have other personality traits.
To call someone in the US a muppet would only make one wonder, 'Which one? Miss Piggy? Fozzy? Beaker?'
They are genuinely going to march with Muppets (capitalised and trademarked) to protest the cutting of funding for PBS (who hosts Sesame Street)
A grassroots protest to save PBS funding, dubbed the Million Muppet March, is scheduled for Saturday, Nov. 3 -- three days before the presidential election -- at the National Mall.
Mitt Romney's threats during the first presidential debate to cut federal subsidies for PBS galvanized support for the "Sesame Street" network -- including Michael Bellavia and Chris Mecham.
Trademark (U.S.) Sept. 26, 1972, claiming use from 1971, but in print from Sept. 1970. Name coined by creator Jim Henson (1936-1990), who said, despite the resemblance to marionette and puppet (they have qualities of both), it has no etymology; he just liked the sound.
As for the question about the slang, I would think calling someone a muppet will be negative in any language, denoting a person with someone else's hand up inside making them do their bidding
A person who is ignorant and generally has no idea about anything.
As a dual citizen of both countries I can say beyond a doubt there is no American equivalent of the British slang word muppet. Same goes for plonker, wally, jesse and many many more. The British are very creative.
For an example of the British usage of "muppet", see the film Lock, Stock and Two Smoking Barrels, in which the character "Hatchet" Harry Lonsdale (played by P.H. Moriarty) remarks, "I don't care who you use as long as they're not complete muppets".
"Moppet" is an alternative spelling for "Poppet" (see the Wikipedia article on Poppet) and has nothing to do with "muppet". If a chap calls his girlfriend a poppet he will probably be in her good books; if he calls her a muppet he will regret it.
'Million Muppet March' Planned To Defend PBS After Romney Big Bird Comments
Plans to save Big Bird, the fuzzy yellow character on U.S. public television's "Sesame Street," from possible extinction are taking shape in the form of a puppet-based protest next month dubbed the "Million Muppet March."
The context explains the use of the term Muppet here.
The term Muppet was invented by Jim Henson at the beginning of his career to describe his puppet act. It is sometimes claimed, and refuted, that Henson created the term as a combination of the words marionette and puppet. Henson used the Muppet name to define the characters in his productions, and to distinguish his act from those of other puppeteers.
In the U.S., the term "muppet" is also used in the financial product sales industry (read: stock brokers) to refer to a "mark"; a ignorant individual who can be easily exploited.