Why does Japan have lots of guard rails in high places? Because the last time a fat man fell from high up millions of people died
Are you making fun of the civilian victims in World War 2?
It just a joke chill out. I'm not hurting anyone.
The only way this joke makes any sense is if it's referring to the Fat Man bomb that was dropped on innocent civilians in Nagasaki.  This trivializes the terrible harm that they suffered as a result, and as such, it is no joking matter.