Tbf whenever you lose you only lose like 10-20 mmr if you don't have an extremely terrible score. Whenever you win, you can gain like 20-30 in one game even if you're around the bottom of the scoreboard. You simply gain more for winning and playing mediocre, than you lose for losing and playing mediocre. If you play well you can gain 60-70 mmr in one game, and you only lose like 5-10 mmr if you lose with a decent score.
Edit: this can serve as an example:
played like shit and lost (-15)
played like shit and won (+21)