Who Benefits?
Some may argue that smurfing really is not ethical while those who do smurf will obviously argue the opposite. But both sides will agree that smurfing is a perfectly normal practice that many players take part in, whether that be to make themselves feel better or to attempt very high win rate accounts that they can show off or sell.
The Smurf
Generally those who smurf do it for one of a few reasons, their main account was banned, they can no longer compete in their main account's elo, or they just want to have a high win rate account they can show off to their buddies. Whatever the reason maybe smurfs are the bane of many low elo players trying to climb the ladder. The awful feeling they have when they check op.gg just to see their opposing laner has a 90% win rate. At one point we have all felt that, and while it may suck to lose for some it is an opportunity to play against this significantly better player and try to improve.
The Sad Opponent
Being on the other end of a smurf sucks. No one will disagree, even if you think it is fun playing vs a player that is much better, higher elo, and is going to flame you it still sucks to lose lp! However Riot does not ban players for smurfing, they will ban elo boosters who log into an already made account to help them achieve a higher rank.
Who really loses?
The LoL community is quite separated when it comes to the issue of smurfing. As could be expected the high-ranking players who commonly smurf to show off or have fun will argue that smurfing is a completely acceptable thing to do.
While low-ranking often newer players will be extremely angry to have to play vs players that are much better than they are. With many of these players calling for these smurfs to be permanently banned.
Verdict
The verdict on smurfing being ethical really comes down to one thing. Everyone is willing to hate on those who smurf until they themselves are good enough to smurf.