Muk question

You just answered your question.

Not really a fairy killer anyway, Clefables have psychic moves and Wigglytuff is just trash now

As you said, a good fairy counter. Probably the best there is.

He's good vs grass as well, as he resists it. But fire is better, so you could just use Arcanine.

Muk is quite good at defending too, although he has some weaknesses. That's no reason to not use him though, as most mons have clear counters.

I would use him purely for his badass animation of flicking the finger and throwing a trash can at his opponent!