In this game, I think that a 50/50 is essential in order for a character to be competitive. The problem with your Killer Frost comparison is that Frost basically had one good option for footsies: slide. Her mobility, which was a general problem in Injustice, was also poor due to her bad walk speed, slow dash, and bad jump. She basically had to hit you in order to set up her 50/50.
In MKX almost every character has a long-reaching footsie tool and mobility is REALLY good in general. Even outside of the run button, walk speeds are much better than they were in Injustice. What this means is that every character with a 50/50 is at an immediate advantage because they have a much easier time getting in and mixing you up than characters in Injustice did. It is much easier for characters in MKX to set up their mixups because of the mobility.
Having said all of that, I still think it may be possible for a character to be decent in MKX without having a damaging mixup. The only problem is that characters who do not have mixups have basically nothing else to compensate them. Shinnok is most likely supposed to be a zoner but his projectiles are really negative and risky to do, which means that he is left with average footsies and no mixups.