Yes, most of the continent was considered the "frontier" at some point. There are few areas of the country Disney could have chosen that WEREN'T the frontier... and New Orleans is one of the bits that wasn't!
However, that's all besides the point, because theme parks in general and Magic Kingdom in particular operate on the level of hyperreality, the realer-then-real. The "frontier" is the frontier of the popular imagination, the red rock and deserts and tough little towns of John Ford and Hollywood. It's the "frontier" of a kid dreaming on Main Street in Marceline, Missouri. If you have to explain the concept, if you have to cite the works of James Fenimore Cooper to attempt to redefine a concept that is fundamental to the popular imagination, you've already missed the point of Disney World. (And, as explained above, your explanation would be wrong anyway).
There is no way Cars in a national park or New Orleans in the 1920s can fit into the "frontier."