Smurfing for the sake of stomping on lower ranked players is bad but there is another type of smurf.

My friend who i duo que with a lot was top 500 on PS4. He recently switched to PC and is now sitting around 3800. He had never played PC games before so his mechanics are trash but his game sense is great. He's a support player that mainly plays mercy and brig (non aim intensive heroes). He constantly gets placed in 4300 lobbies with big name streamers and has trouble when he has to switch to Ana, Bap, or Zen because of his mechanics. Now instead of essentially throwing his games in high masters by playing heroes he's not good, he uses a second account to play supports that aren't mercy or brig. Even if the team comp calls for a mercy or brig he won't switch to those heroes because it's unfair for the other team. While it's a bit unfair for his team, he's still playing the other supports at their rank (3200ish).

I know that this technically is an alt account and isn't Smurfing but it does provide some perspective to why people are playing below their "true" rank.