There's no evidence that Mewni is so traditionalist.
The King has no real power, so who they are is of little consequence in itself, outside of the potential for political arrangements.
The Queen leads the kingdom, as its ultimate ruler, administrator, and greatest defender in a crisis. Star is the crown princess who will assume these responsibilities eventually.
The Butterfly Kingdom is already the dominant power on Mewni and no clear enemies among the other kingdoms, so there's not much to be gained from political marriages.
It's entirely reasonable to assume Star is more-or-less free to choose her suitors.
Royal status aside, Moon herself chose to marry River over objections from both of their families who still hate each other decades later.
Marco is just a kid from a backwater dimension, the worst anyone can say about him is maybe she could have picked someone more prestigious. But he's been a faithful and competent defender and friend to his princess, and he's probably the first human ever to clear Hekapoo's trial. With Moon and River's personal approval it would be very hard for any objections to be raised.