So what?
How that player experiences the game has little to no impact on how you experience the game. If you decide to be salty about them playing the game by the rules that's on you.
What if that person worked hard to assemble a team JUST to solo that Muk? That's a great accomplishment for them and has nothing to do with you. Maybe they jumped in on a raid with a friend and had a nice time together taking it down. That's cool. Either way, the time it took you to get a Muk has nothing to do with them or anyone else, really. Quite frankly, the fact that you're annoyed by someone getting a Muk at this point in the game's existence is laughable.
Is it "fair" that they can have a level 20 Muk at level 16? Maybe not. But what difference does that really make? They'll have to gain 3 levels just to power it up once. Again, it has little to no impact on you.
It's a game that's meant to be played for fun. There's no true end goal or way to "win," so it's all about achieving personal goals and having fun. If it's not fun for you to watch other people have a good time — especially while they gain no advantage over you — maybe this isn't the game for you.