> Now, increasingly, the context seems to be that it's unacceptable/unfair for some reason. This feels wrong to me.
The context in which "smurf" is used these days is very different. But if you try to look at it from the original MMO definition of that term, that would explain why it might feel wrong to you.
In an MMO, people refer to using many characters on one account as smurfing. That's not the case with competitive multiplayer games, where it refers specifically to using a separate account to get (initially) put in a matchmaking pool with lower-skill people, in order to "have fun" obliterating them. As opposed to playing with people of your skill on your regular account with a calibrated MMR.
In an MMO, having a smurf doesn't ruin anything for anyone else. It is just a way to play a different character class, how else is a person supposed to do it? Asking users to restrict themselves to playing only one character class (e.g., mage) forever out of many available in the game is just strange. You still have to level up that smurf character, collect appropriate gear, etc.
In a competitive multiplayer game, like CSGO/League of Legends/Overwatch, etc., none of this is the case. You can play any character on one account. You aren't expected to play one character forever, that would make zero sense in a game like League. What smurfing refers to here is existing players with high MMR creating a separate fresh account, and tearing through new players (because new accounts, rightfully, get placed in a lower MMR matchmaking pool, along with other new players). It is basically an equivalent of Mike Tyson disguising himself perfectly to look like a newbie, and then going to a local boxing gym and just beating everyone there within an inch of death in the ring.
This is the context in which smurfing is being discussed these days as a negative, and I agree with this stance. Your example of smurfing in an MMO, however, is perfectly valid and causes no harm, so it is totally fine.