I don't know where this idea came from that mixups and 50/50s aren't a valid tenet of fighting games -- but they are. You will find them in most of the major fighting titles, going back years to decades.
Not all 50/50's area reactable. Usually the way they're dealt with is to make some kind of drawback -- speed (they get beat by faster normals, even if they're not fast enough to react to), safety (they're unsafe on block), or damage (one or both options doesn't lead to a ton of damage). It's not necessary for unreactable mixes to have ALL of the above drawbacks, but they usually have some of them.
The second set of possible drawbacks involves the character's overall toolset vs. the meta. For example, maybe the character is built to be oppressive up close, but doesn't zone well, or doesn't have the greatest mobility, or has some other aspect that puts them at a disadvantage before they're in range to mix.
In addition, MU-wise, there can be other meta-specific drawbacks, like range (you need to get into a range that puts you at a disadvantage vs. another character's normals/tools, for example), or hitboxes (the normals don't anti-air well if people jump in on them, etc).
Just like every other tenet of fighting games that can be annoying, it's up to you to 1) Find ways to keep the opponent from getting into the situation that's most disadvantageous for you in any MU and 2) Punish your opponent to the fullest whenever you block something that's negative enough or bait a whiff. The fact that most Sub-Zero (and since she was mentioned, Jade) players aren't getting very far vs. good players of other characters, even in online tournaments, tells you that both of these things are happening with regularity.
This is Fighting Games 101. If you don't want to deal with it, you should probably pick up Among Us or Rocket League or Fortnite and call it a day. Good luck.
