And it's not like "apartheid is bad" is much of a political statement.
It's a fine thing for the show to tackle because it's gotten so much buildup (without ever being the real focus of the show, even now), Star as a character has every reason to be concerned about it, and so far it's being handled pretty realistically.
Star can abuse her authority as a princess all she wants, she's not going to change everyone's mind about monsters overnight. I'm sure by the end of the show Mewni will be an integrated society (because holy shit what a message that would be if it wasn't) but she's going to have to work and suffer like hell to get there, like every other gain she's made in the show. And she won't be able to to do it on her own.