Playing against lower-rated opponents does not necessarily hurt your rating. In fact, in some cases, it could boost it. It all depends upon the percentage of games you win and how much lower the opponent’s UTR is than your own. If the opponent is rated too far below you and you win the match easily, winning a high percentage of games,that won’t lower your rating, because that is the expected outcome. It can actually even raise your rating if you win a higher-than-expected percentage of games. However, if you play a close match against that opponent, that could lower your rating.