OP doesn't mean good but easy to use OP stands for overpowered. Superman has too many really good tools compared to the rest of the cast. Like when something's already a good character, and they throw some unnecessary bullshit on him too, that's what makes you OP. Like his trait. He gets like 70% damage with trait on, but that's not enough so they make it armor breaking.
I understand what makes a character arguably OP. The primary argument being provided is that he is unacceptable unlike Kabal because of ease of execution. Which is a flawed argument.
As to what tools Superman has, I agree they are good. I don't think they are OP however. Other characters do comparable damage, with and without trait. Have better wake-up attacks. Etc.
Superman is top 5 in my tier list right now. But he isn't the top of the top, I expect him to slowly move down the tierlist, and I expect certain other characters to slowly move up (characters that are lower on the list due to high execution requirements and lack of top-player interest specifically).
We as a community don't know shit about how this game will look in a year or two assuming it survives that long - but based on historic precedence, we will learn some new tech, and it will disrupt the tierlists as we understand them. Superman will never be bad. But I don't think he as the kind of dirt that will rule this game down the road, dirt like unblockable setups, etc.