There's not really enough information in the public domain thus far for me to conclude that this is a typical example of an elected prosecutor with future political ambitions choosing to overcharge people who are currently very unpopular. But I am leaning towards that conclusion.
I really don't think that, based on the information that is in the public domain, that there's enough proximate cause to link "they bought the gun" to manslaughter. Some states have laws that explicitly allow parents to be prosecuted should a minor access one of their weapons and subsequently cause injury or death; Michigan does not, so this prosecutor is piggybacking on the manslaughter charge as a vehicle to penalize the same conduct. We'll hopefully see what appellate review says about this move.
I don't think judges or prosecutors should be elected at all. The point of having a judiciary is to insulate, to a certain extent, the reading of the law from the winds of populism.