Not an expert, just a burly guy who's done some weight training.
Accuracy,training, and practice are all more important, IMO, but enhanced upper body strength certainly doesn't hurt.
Mike OTDP listed 3 excellent exercises.
One other: take a broom, shovel, hoe,etc, and grip it tightly at the very end of the handle. With the far end of the tool resting on the ground, and your arm fully extended, twist the handle till the tool is raised off the ground to the height of your chest. Do the next set of ten with your weak hand. Then go back to the strong hand, do the next set with the tool facing the other direction. Do the last set with the weak hand in the other direction.
Once again, only opinion, but if you're shooting 9MM pistols and heavier, you probably want enhanced
upper body strength to help with control, aim, and recoil.