Basically, whatever the culture decides is a salient personal characteristic to draw the social hierarchy around.
In America and all other human societies I’m aware of, having an “outie” belly-button is uncommon but is not a salient characteristic anyone cares about. Those with outie belly-buttons are not regularly segregated into undesirable neighborhoods, regarded as inherently criminal, deprived of opportunities, more often subject to police harassment, etc. etc.
However: in the society of the Sneeches, having a Star-Belly confers greater status and social worth. Seems like their equivalent of an “outie” belly-button. A silly distinction, which is precisely Dr. Seuss’s point, I suspect.