In all seriousness, given that Steppenwolf is clearly shown wearing armor throughout all of Justice League, there’s a strong chance that is just part of the under-layer that looks more flesh-colored from afar and due to the lighting in this scene.
Hey, if exposing his booty makes him a better warrior, then by all means, moon the DC heroes.
However, in my head, I now like to think that Steppenwolf felt that to be at the top of his game, he needed his ass to not feel restricted, hence why he opened that “window.”
Because Steppenwolf was being pulled down by Wonder Woman’s Lasso of Truth when these screenshots from Twitter user Sporaw were taken, the moment looks blurry, but nonetheless, it does look like the movie’s main antagonist decided to go into battle bare-assed.