Sanger Zonvolt summed it up perfectly.
Muk is what, NU, for a reason. It has very lackluster stats, and it's abilities are also really bad. Just because you won a few rounds with some random pokemon, doesn't mean it's good. It means your opponent sucks. The bottom of the OU ladder is known to be complete trash. I've swept an entire team with a sunkern before. That doesn't mean sunkern is good, and if you won a few games with muk, congratulations, but that still doesn't change the fact that muk is complete trash.
Muk is what, NU, for a reason. It has very lackluster stats, and it's abilities are also really bad. Just because you won a few rounds with some random pokemon, doesn't mean it's good. It means your opponent sucks. The bottom of the OU ladder is known to be complete trash. I've swept an entire team with a sunkern before. That doesn't mean sunkern is good, and if you won a few games with muk, congratulations, but that still doesn't change the fact that muk is complete trash.