Omniman had to be discreet because he knew the heroes of his Earth would never side with him. Injustice Superman got most of his universe on his side. He isn't a buffoon because he didn't need to use the same methods as Omniman.
This is like saying Avatar wasn't about Aang or Star Wars wasn't about Luke Skywalker. Just because other stuff is going on doesn't mean there isn't a central character to the story.Injustice isn't really about evil Superman, it's a typical dystopic future power fantasy about a resistance vs a regime.