You wouldn't even need a marketable villain in a film with this many well-know heroes, except that the heroes themselves are a fucked up mess.
Gadot is forced to ignore all of her character development in Wonder Woman and go back to being the reluctant Earth-abandoner, still pining for the Pine.
Superman has HOPE just tacked onto him in the opening moments, the story aspects of alien invasion are forgotten, fuck they don't even refer to it, he's just uncontroversial OMG HOPE guy now out of the blue sky. But at least he's consistently a dick when not soothed by Lois.
Batman, holy fuck did they bugger Batman. He's alternately maudlin, morose, despairing, snarky, jokey..... He spends the whole movie begging for someone to take over his responsibility for the team. He rails on Diana for not getting over her dead pare-I mean boyfriend. THEN digs harder on her for not behaving like Superman BEFORE Superman, ignoring the idea that he was trying to kill Superman a week ago for being Superman.
Flash is Big Bang's Sheldon on ecstacy, he gawps and goggles at every setpiece, even Bruce's Bat-Warehouse. He's Jar-Jar tier.
Aquaman is a dudebro that gets two meaningful scenes of dialogue, the first of which has been scene with Bruce in Scandinavia. The other is the silly "I'm sitting on the lasso and pouring out my guts." His scene with Mera is him listening to exposition about his own past.
Cyborg is....well he's actually not terribly butchered character-wise, it's like they forgot he was in the movie and didn't write him any stupid dialogue. He only has to contend with looking terrible.
Oh and Irons is in it, as filler occasionally delivwering pithy one-liners.