in Biciclops all of the supposed Cyclopses had the same shade of purple hair, no matter what their skin color was.
That doesn't really mean anything since it was just Alcazar trying to convince her that she belonged to his race.
But your points about the Simpsons are valid... I don't really watch much Simpsons (seen maybe like ten episodes ever) so I didn't think maybe it's just a Groening thing.
People assuming that she dyes it doesn't change the fact that she doesn't dye it. Her parents have purple hair too, so it seems like a mutant genetic thing.