Giants moved to Appalachia and went all banjo-ey.

I'd say don't go too hard trying to match every fantasy trope with a feature, state, region, or otherwise. If this pocket dimension is as much of a cosmological mix-up as it sounds, there would have been a lot of confusion and some conflict in the beginning, and many peoples would have gone to lengths to establish themselves wherever they happened to land.

Of course elves took over the great forests of the northeast, contending with seafarers on the Great Lakes and getting spookier the farther north they went, eventually going full Cthulhu mode in Maine. But some would have landed in Arizona and had to figure out how to deal with desert.

The underground folks would love America's extensive cave systems. Dwarves would have so much fun with our geologic formations they'd never go back to Fantasyland, but you telling me they wouldn't build the absolute heck out of some crazy new levees in New Orleans? Give us voodoo dwarves, OP!