You certainly have a point, but note that the legal standard for self-defense is not entirely dependent on the severity of the injuries actually suffered. If Zimmerman reasonably believed that he would suffer severe bodily injury or death if he did not resort deadly force, then he has a legally sufficient self-defense claim. If, when he fired the shot, he reasonably believed that he was defenseless and had no means of escape, then he was not required to assume that the beating would stop short of "death or serious bodily injury," even if he had not already suffered serious injury at that point.
The issue is whether he reasonably feared death or serious bodily injury, not whether he actually suffered serious bodily injury. Obviously, if his injuries were in fact severe, that would pretty much prove that his fear of serious injury was reasonable. But just because his injuries were not severe, that doesn't establish that he was unreasonable in fearing that he would suffer serious injury or death if he did not use deadly force. Many people who kill in perfectly legitimate self-defense do not suffer any actual injury.
That said, any evidence that he is being less than honest would certainly tend to discredit his entire account. So if it can be shown that he deliberately exaggerated the extent of his injuries, that would tend to cast doubt on his credibility in general. But in that case, what would matter would not be the fact that his injuries were not severe, but rather the fact that in exaggerating his injuries he would have exhibited dishonesty.
