Trump's criteria for supporting anybody is unquestioned one-way loyalty to him personally and how much they suck up to him (e.g. Mike Pence, Devin Nunes), or in the case of foreign autocrats to "say nice things about him" (e.g. Putin).
Once upon a time, for a moment, Trump knew enough about McCain to insult him, but right now, I"m guessing he doesn't have a clear idea of who John McCain actually was, except that McCain wasn't personally loyal to him and now he's getting a lot of coverage just for dying. A couple more days of all McCain all the time, and Trump will do something really stupid to get back on the front page above the fold.