It feels as if League always turns into a smurfland in the last two or three months of every season. I had a significant larger portion of lopsided matches in the last few weeks of Season 9. These matches' results were usually determined by the one or two people who had been doing exceptionally well judged by the match history (e.g., 90% overall win rate). According to Riot, smurfing is an acceptable behavior as long as it's not some sort of elo boosting service (e.g., selling the leveled up account to someone else). But official rules like this oftentimes are static and do not reflect community understandings. Players enter a match with the expectation of having a fair game where each side has roughly 50% chance to win. But smurfing engenders a greater sense of loss of control for the rest of players in the same match with smurfs, whether they are on the winning or losing side. Smurfing might also have a cascading effect where players having less fun because of smurfs start to create their own smurf accounts. This could become a concerning issue for the game developer if new players are scared away or verbally bullied by smurfs. In a rapidly-growing community, smurfing might present less of a problem than a 10-year old game like LoL. Although without Riot releasing their data, it is hard to tell if LoL has saturated the market or if the player base still has steady growths.
Fairness can be seen as a shared resource, and smurfing is a free-rider problem. Although hard to quantify and measure, each player has their own sense of fairness developed through their everyday gaming experience. Such sense is communicated and circulated explicitly through discussions (or complaints) about issues related to matchmaking, smurfing, climbing, etc., and implicitly through in-game actions (e.g., a player titled by smurfs started to behave aggressively).