The biggest problem with Grand Master Sub-Zero no matter how good or bad he currently is. Very few people actually enjoy the way they have to fight against him. Clone is always making him go backwards and putting up a wall in front of the opponents face, and they just may have been trying to hit him during it and got frozen, into a knock down into more ice walls into mix ups into bleh. It's incredibly frustrating to get killed by someone doing that through out the match.
Now don't count me as whining I'm not, but put your mindset of that into a "Filthy" Casual. You suck at the game, and every time you try to get in to hit the Sub's you run into a clone. They will rage, they will complain, they will be pissed off. They are not interested in leveling up, they just want to have a fun time.
If their experience with the game is constantly run away Grandmaster Sub-Zero, then their mind set on Mortal Kombat as a Franchise is darkened. Now you are someone from NRS, Millions of people play your game, and are excited about each and every new game. If the casuals just remember getting pissed off in the last game, they may not want to buy the next. And only I'd say about 25-30% of MK's fanbase are tournament players, or are part of the FGC. You'd want to cater to the 70% no matter what. This is the biggest reason GM Sub was nerfed. That's all.