Yeah, if you're way higher elo than the other players in your game, you gain noticeably less MMR for winning and lose noticeably more for losing. I did a lot of duo queue with someone that was silver V when I was gold I, and eventually came up with that, in doing that, I had to maintain a 53% win rate just to stay at the MMR I was at from doing that, which only seemed to be like a gold III or so MMR, even though the league system had me in gold I.
And it does this in normals, too, not just ranked. If you want to get a high win percentage, kda, etc., duo with people worse than you. But if you want to raise your MMR it's going to be a lot easier if you don't queue up with people worse than you.