Any attempt to bring a Mii sharing the German dictator's name online for Mario Kart Wii will be met with the following error: "You cannot access Nintendo Wi-Fi Connection using this Mii name. Please change your Mii name and try again." Presumably, you should still be able to race with a Mii bearing Hitler's likeness while using a different name. You know, if drifting through Moo Moo Meadows as the architect of the Holocaust is your idea of fun.
According to initial tests, Joseph Stalin, Benito Mussolini, and several other dictator jerks are not currently banned. Really, though, when was the last time you ran into someone using a Mii of Robert Mugabe.