Can there be humor about Hitler and the Nazis?
The movie and recently revived stage play, "The Producers," by Mel Brooks, suggests that there can be, and that the humor can be part of the battle against Nazism. During the Nazi period and WWII, humor was used as to motivate opposition and to support the war effort.

One example is Charlie Chaplins "The Great Dictator" (1940).

Another is a skit (1942) by Spike Jones and His City Slickers (Spike is the one of the left). Note the misspelling of "Fuehrer" as "FueRHer" and the obscene gestures, which at the time would not have been acceptable in public performances for other purposes. (Click the picture to start the movie clip in another window.)