MM game quality question

Why MM games have a bad quality, or is the quality acceptable, or the % is showing nonsense?

My last 20 games ( 53% min, 84% max, 67% avg. )

This is probably because some people are still new to the MM system therefore it does not fully know their rating.
Basically its still learning meaning there may be some lower balance or higher balance during this time.

Ras Boi's save lives.

Because there is a trade off to requiring higher quality, which is that you will be sitting in the queue for a very, very, long time. This is exacerbated by FAF having a pretty small player base, so the algorithm has to be pretty forgiving in terms of game quality so that any matches can happen at all. Generally the more players are in queue the higher you should expect the game quality to be.

As @NoOneCares said, there is also the fact that if your rating has high deviation (I.e. you are new) then the maximum possible quality you could get even against someone with exactly the same rating as you tends to be very low (as low as ≈40% for brand new players).