I assumed the MMR was supposed to be around 10~20 difference between all players in a game, but I've had people with 56% win rate vs people with 46% winrate. I've noticed games where one team has an average winrate of 53.6% while the other team has an average of 50.4%. Hell, I had a game earlier where my team won in 17 minutes with 20 kills and the enemy team only had 1 kill, I don't think that's a fair match.
Riot, how does this shit work? Maybe the community can help you fix it if we knew better how it worked.