Unfortunately, that doesn't work in any games where you play on a team, because then everyone else on your team is playing with a lower-skilled player, which makes the game much harder (and usually less fun) for them.

The only way you could implement this feature in a fair way (for both teams) is if you allowed an "MMR boost" when your entire team is a single party. And that would be fair! I'd support that feature, even if I never use it.