I'm puzzled by the notion that no one may speak ill of the dead. We don't have any problem speaking ill of people like Manson, Hitler, Whitey Bulger, et al., do we? McCain may not have been a mass murderer, but he still was a piece of crap.
My only objection to Trump criticizing him is that it's a waste of time. Please, el presidente, do something more important with your time.