It makes sense if you're Irish or British. People here who are blond and blue-eyed but whose ancestors come from Ireland assume they have Viking ancestry, and they're probably right since the Viking era was the main time Scandinavians mixed into the Irish gene pool. It doesn't have white nationalist connotations: the alt-right probably identify more with Celtic iconography if anything.

If you're American, it doesn't make as much sense, because Scandinavians and Germans have been coming to America for hundreds of years.