Yes in theory, but most rear bumpers are rather flimsy. They're designed from something to hit them horizontally, not to have weight bouncing vertically up and down on them. It all depends on how strong the bumper, and its connections to the frame are so that the constant vibration doesn't crack something.
And there's always the chance of someone hitting the generator or you backing into something when you need that extra two feet of room.
Several truck campers mount their Honda 2000s on their ladders and then the extended run fuel tank above the generator. It seems to work.
Bill
Nodwell RN110 out moose hunting. 4-53 Detroit, Clark 5 spd, 40" wide tracks, 10:00x20 tires, 16,000# capacity, 22,000# weight. You know the mud is getting deep when it's coming in the doors.