MMR is a measure of individual skill retrofitted to fit into a team environment
And UFOs are spaceships from other planets carrying little green men?
No. MMR stands for match making rating, and is a generic term. Pretty much every implementation of matchmaking systems is different, even when you're talking about something as specific as Elo ratings, which is what you're thinking of, with very few Elo based systems directly reflecting what was outlined by Arpad Elo. A common theme across MMR systems is adjusting player ratings by wins and losses, and altering the amount of points lost or gained based on the predicted outcome based on MMR going into the match. MMR based matchmaking systems are a separate thing from MMR itself, and is a system where you are matched with other similarly skilled players based on your MMR.
MMR based matchmaking is based on the principle that, if your MMR is accurate, your performance should be predictable. The systems don't tend to expect to be able to predict your performance in every match, but are designed to predict your average performance. The changes in your MMR happen in such a way that, if it doesn't currently predict your performance, it eventually will. If you win or lose when you aren't predicted to, you gain or lose MMR fast, and as a result you will be expected to perform at a higher or lower level in the future. It's a self balancing system for predicting your ability to win (also known as skill) based on your actual ability to win (also known as skill).
If you play on a team, you all have an MMR that more or less reflect your skill. If someone else on your team has an MMR that is lower than it should be, you'll probably win, because you were matched based on the MMR they did have not the one they should have. This will raise everyone on the team's MMR, and in that single game a win based MMR system will not know which player caused the unexpected win based on this single match. But it doesn't expect to, or rely on it, because everyone will go on to play more games, and the player that caused the win will keep having their MMR incrementally adjusted upwards, while the rest of the players will see their MMR move back down. It's a system with what is effectively random noise, but built in a way that effectively tests each instance of data to see if it was noise by treating it as if it wasn't.
There are variations of MMR systems that add additional complexity for achieving specific things. Some are really advanced and use specific in-game performance metrics such as points scored by individual players to more accurately reflect individual skill in a fixed team environment. This is largely unnecessary in an environment without fixed teams, but can in certain cases reach an accurate MMR faster. Other systems add a confidence interval to the MMR to facilitate changes to MMR plasticity when players perform as expected consistently, or consistently perform outside of expectations. This avoids unwarranted impact on MMR when you happen to land in a game where another player carries, or from outlier events causing winning or losing streaks, while also increasing the speed at which you initially arrive at an accurate MMR or at which a player can change their MMR when they start to play better or worse. Blizzard is not known to use in game performance metrics for their MMR, but do use an MMR confidence modifier.
Nothing in how any of these systems work relies on a player being in the role with the highest potential impact on the game in order to arrive at an MMR that accurately reflects their skill, or lets them consistently win at 50% while playing with others of a similar MMR. There is no logical mechanism for how your MMR would become more accurate by playing a role that has the potential for having more impact when you are playing with people of a similar skill to you, and when you are the only consistent variable across all of your games. Nor is it possible for any two team game to have a combined winrate of anything else than 50% without draws, or for there to be a combined impact on winrate of more than 100%. However the impact is distributed among players, the average impact per player per game is always going to be the same, making it entirely nonsensical to talk about one game having more relevant MMR because one role has a higher potential impact on winning.
So, here's the real problem: You're talking fucking shit. You're making stuff up, and you're deciding it's true because it sounds right in your head. But it's not true, it's absolute poppycock.