I suppose it is because the Germans think the Swedes are blonde in all the wrong places and vice-versa...
~Theo
It dates back to the end of the Thirty Years War. The Swedes basically won the war in the North, and were a very hard master in many areas.
Growing up “Swede” was a near swear word. Didn’t understand why till I got into genealogy and realized most of my area came from the Northern Baltic shore, and from southern Germany (the Catholics). Both suffered under the Swedes, and carried the distaste to the USA.
The only worse thing was a Prussian.