Yes, an individual right to a handgun is important to military preparedness, because it teaches the citizens how to use weapons, and also sets the tone that weapons are important and a needed use of justified force in certain situations. A well-prepared military is familiar with guns, and an individual right to a handgun encourages the public to be familiar with weapons and their appropriate uses.
Whether or not individuals have handguns will not make the military more prepared. Civilians are not part of the military, so it does not matter if they have guns or not. Furthermore, even if normal people have guns, there is no guarantee that they will be trained to use it.