Muk isn't particulary bad, but this is the special case of bad typing match up.
First of all Psychic type moves is pretty much kills Muk faster than other "walls" such as Lapras and Snorlax who has limited foes counters them in the first place.
Alakazam literally kills muk faster than a speeding train for god sake, even Common attackers such as snorlax with ZH, Exeggutor with ZH + Psy, Slowbro water gun + Psy, and even Rhydon/Golem with MS/EQ literally dancing in his grave.
Muk is good only for pure grass and fairy attackers if they gonna add some to the meta. But in the mean time, Muk is spelled backwards which is sucks.
If you want to upgrade your Muk for defensive go ahead, but this meta will f*** him up so fast coz of his typing only.