Playing against lower-rated opponents does not always hurt your UTR Rating. In fact, in many cases, it helps it!
It all depends on the percentage of games you win and how much lower the opponent’s UTR Rating is than your own.
Here is how it works:
If the opponent is rated too far below you and you win the match easily, winning a high percentage of games, your rating will not lower. This is due to the outcome of the match occurring as expected.
Playing an opponent with a lower rating can raise your UTR Rating if you win a higher-than-expected percentage of games.
However, if you play a close match against that lower-rated opponent, this could lower your UTR Rating.