Considering the 82% in-state requirement and the relatively smaller population of NC relative to CA, TX, and FL, I've always thought we've done reasonably well in USNWR rankings. Consider some of the other "top publics"
Berkeley - 76% in-state (CA population 260% larger)
UCLA - 79% in-state
Michigan - 52% in-state
UVa - 66% in-state
UT-Austin - 80% in-state (TX population 182% larger)
U Florida - 79% in-state (FL population 109% larger)
I am not advocating that NC change its requirements, but simply suggesting that if rankings were the most important factor (and they certainly should not be), Carolina could likely move a bit higher.