MMR is almost entirely based on wins and losses. I'm talking a system that's independent of winrate. Good players can still lose a lot, and bad players can still win a lot, because of their teams carrying or throwing.
And my last two games (as of when I wrote this post), I was 19/5/18 and 10/2, while my team had over 30-40 deaths each time. In the Poppy game, I couldn't contest objectives near the end of the game since the entire enemy team was fed, and we lost teamfights because Vayne was allowed to just farm and catch up when I set her so incredibly behind. The Rek'sai game, I just kind of watched my team repeatedly tower dove and feed while trying to catch my team up again, wasn't much I could do.
Don't know that it was MMR, but I was matched with people who absolutely cannot play the game, while I was doing well, and I still lost. So I'm talking revamping the matching system, so bad players are matched with bad players, average players are matched with their own, and good players are matched with good players.