I have a feeling that over the long haul, there's a tendency for the W/L to be somewhat greater than 1. There are two factors at play here.
First, most players will find themselves rising in PSR as they play. They start at a low tier, and gradually rise towards higher tiers. This means that all the way through this rise, they're playing at a somewhat "too low tier" for their skill level. Thus they have the opportunity to carry in their games -- maybe not much, but at least a little -- and they thus maintain a W/L of >1 through this process. At some point the player will finally reach his "correct" tier and start to win about 50% of games, but all those games racked up during the PSR rise will remain in the stats, and the W/L will remain at >1.
Second, as we all know, sometimes the MM puts you in a game where most other players are not from your tier. They might be worse than you, or better than you. However, as you get an experienced and get to a high tier, the MM can no longer use you as filler in games above your level, as such games don't exist: you'll only be facing players who are either at your level, or worse than you. So again, there's a tendency for you to get games where you can carry, and still maintain a W/L of >1.
I don't think MM quality should affect long-term W/L. Even if the MM sometimes makes lopsided games, over the long haul you should find yourself equally on both sides in those games. I don't think it credible that the MM would systematically work against or in favour of a specific player.
Edited by jss78, 30 December 2015 - 01:52 PM.