That's not really how MMR works in most games. With a large enough sample size you're, ideally, looking to matchup people at similar MMR and therefore, create a smaller skill gap. It's not the case that the matchmaking algorithm tries to put 3 5000s with 3 1000s to evenly match a team of 6 3000s. That's a misunderstanding of what it does to get that 50% expected winrate. Rather, the game tries to find 12 people who are 3000, +/- maybe 200-400 depending on how loose the algorithm is.
The only time what you're describing happens, the matchmaking literally trying to sink a player's MMR with anchors, is at the extremes of matchmaking where there are people far above or below the median that there is literally no one else for the game to have them play against.
There are probably other reasons you're running into stomps but unless Blizzard completely fucked up their algorithm, the matchmaking isn't solely to blame.
