What is the movie guy dress up like Elvis and are robbers?

You're likely thinking of the movie "The Great Muppet Caper" (1981).

In the movie, the Muppets are in London and get caught up in a jewel heist. To help them, Kermit the Frog dresses up as Elvis Presley to distract the thieves.

Let me know if you'd like any more details about the movie!