Muk is more of an interesting/unique defender more than anything (since you don't often see one in gyms). The one big upside to Muk is that it resists fighting, so it'll likely cause an attacker looking to sweep a gym with Machamp to swap to something else (unless of course the Muk has low motivation).
Unfortunately, Muk also gets murdered by psychic types and will only give them a hard time if it knows Infestation/Dark Pulse.
So, if you want a cool novelty defender, then sure, go for Muk! In all practicality though, Muk isn't a stellar defender (or attacker for that matter), so unless it's a personal favorite of yours or a long overdue pet project, then I'd advise saving the dust for more optimal Mons.
