The thing is that what "looks" healthy is culture-dependent; nowadays in the caucasian west, looking lean and tanned is considered a healthy / attractive look, but in other places, times and subcultures having more weight or pale skin is considered healthy (or attractive).
That doesn't prevent it from correlate. If obesity is the bigger threat than looking skinny is correlated with being healthy. If food scarcity is a bigger threat than the opposite.
However a lot of more subtle things like soft, shiny hair or good skin are correlated with nutrition.
Many looking healthy things are literally just your body functioning correctly.