The answer to this must surely be different at different times in history.  There were times in history when German nationalism has seemed to be more of a positive force and other times when it was clearly a negative force.

German nationalism is typically traced back to the time when Napoleon and the French dominated Europe.  In the wake of the French Revolution, Napoleon dominated Western Europe.  This French domination ignited a sense of nationalism in Germans, who did not like being dominated in this way.  This wave of German nationalism can be said to have had positive consequences.  Most particularly, it led to a huge upwelling of German culture.  This German nationalism helped bring about the romanticism of people like Goethe, Schiller, and Beethoven.  It also helped to give rise to the thought of philosophers such as Kant and Hegel.  If we believe that things like music, theater, and philosophy are positive things in human society, then we can argue that this wave of German nationalism was a positive force.

Later, however, German nationalism seems to have been a distinctively negative thing.  German nationalism can be blamed (at least in part) for bringing about two world wars and, perhaps worst of all, the Holocaust.  In the late 1800s, it was German nationalism that led to the creation of the German state and then to Kaiser Wilhelm’s desire to attain Germany’s “place in the sun.”  This helped to lead to World War I.  After World War I, it was German nationalism, along with frustration over the results of the war, that helped bring Adolf Hitler and the Nazis to power.  German nationalism was expressed in the idea that the Germans were a master race and that the Jews needed to be exterminated.  The idea of German superiority helped to bring about WWII and the anti-Semitism in German nationalism helped to bring about the Holocaust.

Finally, we can say that German nationalism has had positive impacts in our own time.  During the Cold War, Germany was split between communist East Germany and democratic West Germany.  The German nationalist desire for unification helped to destroy communism in the late 1900s.  It also helped to cause the reunification of Germany, which helped improve the lives of the former East Germans, whose country had been devastated by the Soviet Union and by the decades of communism.

Thus, we can say that German nationalism has been a force for great evil, but also for good.  Therefore, our answer to this question should vary because German nationalism has had different impacts at different times in history.