It's taken me a while, but I've just come up with a theory as to why people blame the MM for their performance.
I'm a granola tank grinder. I don't free XP modules and I start with a 75% crew. In the early games of my grind, it's almost impossible to carry. The early grind puts me almost entirely at the beck and whim of the MM. Since I can't carry, I'm dependent on the team to carry their fair share (a rare occurrence).
So I only play my new grind tank for the x2 or x2+ in the early play to maximize the XP to get my modules. This makes me hyper aware of wins and losses because all I'm trying to do is clear the 2x. When this takes 4-5 games, things get ugly. I start to blame the MM for not getting me a team that can move off spawn or get over 1k damage at tier 8/9 even though this is typical.
That brings me to my realization about why people blame the MM for their performance. When you are dependent on the team for your win, it becomes easy to you blame the MM when you don't win. And guess what... they are right! It was the MM that prevented them from winning! They don't have the skill to elevate the team to victory so it was the MM that failed them!