It is not possible to answer this question with complete certainty as it is not possible to know how Japanese and American forces would have behaved towards one another in the absence of propaganda. We can never know such things when studying history and can only make educated conjectures.
It is quite possible to argue that propaganda depictions did not make any difference. People who are at war with one another are going to commit atrocities at times. The Japanese culture of the time, irrespective of any propaganda about the US, was likely to produce men who were willing to fight to the death and officers who would force them to do so. In this way, we can argue that propaganda did not matter.
However, it is also possible to argue that propaganda led to more casualties. Both sides depicted the other in very negative ways. Each portrayed the other as in some way inferior. The Americans played on stereotypes of cunning and underhanded Orientals. The Japanese portrayed the Americans as morally weak and lacking the sorts of moral virtues that Japan felt its people had. By painting its enemies in such negative ways, these propaganda efforts essentially gave soldiers permission to despise the enemy and to treat him poorly. This, arguably, led to more casualties by increasing soldiers’ hatred for the enemy.