The Southern states that I'm aware of which have "strong economies" got that way by slashing worker and environmental protections. I don't think that's actually a compelling counterexample.
"the states with the lowest gini co-efficient are the ones that are the least diverse" seems a better fit.
Diversity is just a statistic, it's neither good or bad in either direction.
Not all forms of diversity is strength.
Mixing a wolf with a bunch of sheep isn't diversity, it's dinner time.