If I remember correctly, the earlier games have a much higher impact on your placement. I won the first 5, lost 1, won 2, lost, won.
One game I was carried, but still did my part cause vlad ult op.
One that I lost was out of our teams' hands, outside of the blitz that went to suicide, throwing the game (that we were ahead in).
The other, bot lane and our jungle fed the enemy botlane+jungle extremely hard. When that caught up to me, couldn't really do much. Team dragged it out, so all of our scores in that game look a bit worse than what really happened lol. I had complete lane dominance vs a Riven as Vlad, so I'm not too upset about it. Not much I could have done.
I've personally never had an issue with a single lane being fed. Just afk farm and wait for teamfights, focus fed person. For me, practically works without fail.
If you think lower MMR means your teammates won't be able to accomplish that...it also means the enemy team doesn't know how to play the map and close out the game, letting you get enough items to at least have a shot at winning. Pretty much cancels each other out.