Smurfs are intentionally manipulating their SR/MMR to cheat the system - which is the reason why actual smurfing is a banable offense.
I also can’t confirm that 90% of my games are onesided steamrolls.
At least my subjective perception is, that steamrolls went down significantly with the implementation of Role Q, because the system now can account for separate role SR/MMR and actually knows which role you are going to play.
Also the most common reasons for steamrolls are things that the matchmaker can’t possibly anticipate.
Some players - despite having about the same skill level - just don’t work well together because their best hero picks don’t synergize that good.
In other cases it is people who for some reason are bound to behave toxic towards each other and drag their team down with them.
But the most common factor in steamrolls is plain and simply (bad) luck.
Blaming it on the matchmaker might look like an easy explanation for you, but considering how hard it would be to create a matchmaker that could actually predict with very high accuracy which team is going to win, the most likely explanations is not that the matchmaker is rigged against you, but that you are simply not good enough to maintain the rank that you have gotten into by luck.