If a person of African descent dressed up as a Viking, which is closest to my heritage, would that bother me? Hell no.
That's just no comparison. Africans didn't oppress Vikings.
Not during the Viking age of exploration, but their descendents were. Barbary Corsairs in the 17th Century regularly raided as far north as Scandinavia. Although obviously the bulk of their European slaves were Spanish, Portuguese, and British, there have been a number of documented Scandinavians.
Still, I don't think that's really the best criterion for establishing racism. If that's the case, then every Halloween costume ever sold is racist, because every witch, lion, ninja warrior, prisoner, mullet-wearer, waitress, robot, zombie, sailor, pokemon, grizzly bear, and cable guy could be said to represent some exploited individual.