Lot of nuance here.

First item is that the cost of living differs from state to state, and even proximity to economic centers.

Second item is that some states already do that, and enforce a state level minimum wage, for exactly that reason.

Lastly, 1.5% of Americans make below the federal minimum wage. But to get a more accurate picture, we need to break it down by state, and compare to cost of living instead of minimum wage, which is going to make that proportion go up significantly.