It's easy to claim that the quality of teammates are horrible but then ignore the games where you are the lead anchor of the team.
Matchmaking is never going to be super great. Think about it, at almost every MMR level you're going to have three groups of players.
- People who deserve to be at that MMR.
- People who are moving up past that MMR to settle at a higher one.
- People who are moving down the MMR to settle at a lower one.
The only exceptions to this is the very top and bottom. Then you still have two groups.
You can't stop matches from having all three of these types of players. The best that can be done is to try to accurately move people so that someone that deserves to go up doesn't go down and visa-versa.
But League of Legends is a team game, so that task is much more difficult since you win and lose as a team.