I think those were supposed to be Pennsylvania mountains.
Chimino certainly wasn't the first director to shoot stuff that was supposed to happen back east out west (in this case the Cascade Mountains of Washington State).
You could go crazy if you let the California light and scenery in every movie that's set somewhere else get under your skin.
The majority of the film was shot in Ohio & Pa. http://www.movie-locations.com/movies/d/deerhunter.html#.V3xEgDW3k6Y