First, please do not distort the true. Kerry implied the uneducated join the military vis-a-vie the educated. He did not say people in the military are stupid.
Second, It seems pretty clear to me that what Kerry was trying to say, and admittedly did not say well, is that the burden of military services is disproportionately falling in the poor and and poorly educated. Unfortunately for those who wish to attack Kerry, this is true. Obviously, there are well educated people in the military. However, on the balance, college graduates do not join the military, but rather take jobs in the private sector.