A smurf is specifically a player that is keeping a lower-ranked account around in order to enter lower-ranked games as a far better player than the system accounts for. Hence the name “smurf” to imply a smaller, usually-throwaway account.
Because Overwatch will always pair you up against teammates based on your MMR, and your MMR is decided based largely on the few heroes you happen to have practiced and played with, it is difficult to try and practice heroes you normally wouldn’t. If someone wanted to practice Sombra for the first time when their MMR is skyhigh because they’re exceptionally good as McCree, it’ll be hard to find an environment where you can actually learn the character without getting stomped by opponents that simply aren’t handicapping themselves the way the player in question is. So it does make sense, at least for a lot of people, to buy another account and start fresh so that they can more organically learn the ropes behind a character.
Personally, I would never buy a second account for something like this, but it’s understandable why other people would. The game doesn’t make it very friendly for people to try out new heroes or explore different aspects of the game, especially those that get caught up in higher MMRs. As long as the alt account is playing genuinely, it’s no different than if someone started playing Overwatch for the first time, but due to so much experience with other FPS games, they’re able to dominate their first few series of games. Yes, this alt account does have a steep advantage, but that advantage is quickly dulled down once the matchmaker realizes they’re not actually newbies.