This set should only be used on full stall teams, as Muk will lose too much momentum for the team otherwise, and it doesn't hit quite hard enough or fulfill a role useful enough on balanced and offensive teams to warrant a teamslot. Muk's foremost role on the team is as a Grass-type counter, particularly against Rotom-C, which can otherwise give stall teams some trouble, and the moveset helps accomplish this. Poison Jab has good neutral coverage in RU, the poison chance is nice, and it absolutely demolishes Rotom-C and lands a good hit on other Grass-types as well. However, it should only be used if the Pokemon you predict to switch in isn't more harmed by Toxic. Fire Punch puts the hurt on Amoonguss and Roselia, but it's more useful for luring in and KOing Durant, Escavalier, and Ferroseed, which are all very dangerous Pokemon to give a free turn to. Thunderbolt's main role is to 2HKO Qwilfish after Stealth Rock damage, and it gets a decent hit on Moltres and other Flying-types. The last slot will usually go to Toxic, which deters a lot of the Pokemon that will generally be switching into Muk such as Slowking, Mesprit, Rhydon, Quagsire, and Gallade. However, Haze is usually a requirement on stall teams, and if you're not using Cryogonal or another Pokemon for the role, then Muk fits the bill well. Unfortunately, most of the Pokemon that Haze is most useful against can threaten Muk with their STAB moves or OHKO it if it switches in against the likes of SubCM Uxie, Mesprit, Sigilyph, Gallade, and Kabutops.
Sticky Hold is needed so that Muk can absorb Trick from the most common users of it in the tier, particularly Rotom, Rotom-C, and Manectric. Poison Touch might be tempting to use, but Muk usually can't risk normal poisoning something it'd rather Toxic poison such as Tangrowth and Slowking. A fully specially defensive EV spread takes advantage of Muk's typing and stat spread the best, and even with Defense investment, Muk will still usually be 2HKOed by most physical attackers. Don't bother trying to beef up Muk's Attack either because it needs all the special defense it can get, and investing in Muk's offense is only efficient if it becomes an all-out attacker.
Gunk Shot can be used for a gigantic increase in power and offensive presence, which can also force more enemy switches and rack up entry hazard damage. However, the accuracy will more often than not bite Muk in the slime and force it to switch instead; granted, this isn't as punishing as it is for the opponent, since full stall teams should usually be able to get off a Rapid Spin and have Wish support. Thunder Punch can be used over Thunderbolt if Qwilfish isn't a concern for your team, as it does a bit more to Sigilyph, Moltres, and Kabutops, usually forcing the former two into using Roost, which will open up a free switch for you. Sleep Talk can be used as well if you want Muk to be a complete Grass-type counter, but Muk is a lackluster sleep absorber, as it doesn't pose much of a threat while asleep.
As Muk should only be used on full stall teams, its teammates should follow suit as such. A spinner is needed, since while Muk absorbs Toxic Spikes, it's still susceptible to Stealth Rock and Spikes, meaning Cryogonal makes a good partner, and it can also provide Haze support. Because Muk has no recovery, it appreciates Wish support immensely. Two really good users in Clefable and Alomomola are available, and both have good defensive synergy with Muk.
Steelix and Aggron have a free switch into Muk, so teammates are needed for them. A Steelix of your own or Poliwrath are good choices; Qwilfish also does alright in this role despite its frailty and can provide Spikes or Toxic Spikes once it's in. Magneton works here as well, particularly Choice Specs Magneton, as it can trap Steel-types and obliterate most of them with its STABs and Hidden Power Fire. Sigilyph laughs hysterically at Muk, so a counter to it is needed too. Spiritomb fits best on stall teams, and it can beat most variants of Sigilyph; Haze Cryogonal also works OK here as well. Most other threats that will generally be switching into Muk are maimed by Toxic or its coverage, so simply providing good defensive switches is all that's really needed to wear down the opponent over time and secure the win for Muk's team. Pokemon like Clefable, Amoonguss, Tangrowth, Steelix, Alomomola, Slowking, and Uxie are all classic RU stall Pokemon that cover a huge variety of offensive threats within the metagame.