It was party mmr, I think each team were both a group of 2 and a group of 3.
I don't mind the loose of mmr itself, I'm long past caring about my own mmr. It's just that I expect the matchmaking to try put two equal team against each other, even if it does not automatically mean the match itself will be balanced, at least it increase the probability it will be. Same reason I don't like when the game somehow put a team of two premade against a premade of 5 ( rare, but it happen ).
I usually don't look too much into both team rank, but this time the disparity was so obviously onesided it just felt like the matchmaking didn't care at all.