"Croat" is not a slur, just what people from Croatia are called.
I don't think making fun of Croats for their country's past is any more insensitive than any other instance of bullying someone for their nationality.
If anything is a sensitive topic its not as much their role in WW2, but rather the much more recent Yugoslav wars, and the still existing tensions between Croats, Serbs and Bosniaks.