There is some Disney / Muppet stuff in American History. There is more than Kermit. Miss Piggie, Oscar, Bert, Rowlf, the Swedish Chef, Red Fraggle, Boober Fraggle, Bert, Earnie, Elmo, Fozzie, the Count, Grover, and many others are there.
There's a Dumbo from Disneyland on display there, as well as one of the mouseketeer hats worn by one of hte original mousketeers. A Lion King mask from the broadway musical is on display, as well as sheet music from Pinocchio, and a laser disc (which I actually own) of Fantasia. There is no real "exhibit" about Disney, but if you keep your eyes peeled, there's references all over the place.
There is also a small display of Disneyworld memorabilia next to the TSA exhibit (no, I'm not kidding, that really exists) in Air and Space.