I think the majority of German fans don't identify Nico and the Mercedes team as German. Nico is a "passport German" unlike, for example, Michael. And the team is German only in name.
I think that's also why the new circuits have struggled to fill the seats - the local population have no local interest. We have to remember that for many sports fans, it's the human connection that drives their interest. I bet most people who attend F1 races are there in support of a driver and that driver will usually share their nationality. Except in Italy where it's Ferrari that is supported of course.