While I agree with your conclusion, OP, the premise is a little hyperbolic.
SS doesn't look like a mediocre game. Graphically, it's really strong. The production values are through the roof. It's just that the entire concept, i.e. Suicide Squad, is just so fucking dull and uninteresting.
Out of DC's entire roster of A-list superheroes, they go and make a game featuring a selection of characters that nobody fucking likes or wants to play. If they really wanted to make a villain-focused game, they should have made one about someone like Deathstroke ---but even then, don't waste Rocksteady's talents on that shit.
I don't blame Rocksteady. It's all WB's fault. You just know WB forced them to work on this shit. Because there is not a world where given the choice that Rocksteady would have selected to work on a SS game of their own volition. WB has been trying to make SS a thing since forever. They made two films plus that Birds of Prey abortion to keep trying to push this fucking vapid shit onto consumers.... and we don't give a shit, WB!!!! Get it into your thick fucking skulls.
I've said it many times, but a gritty TMNT game by Rocksteady should have been the obvious choice from WB. It would have catapulted the series back into popularity among the masses, sold fucking gangbusters and rejuvenated the brand so that it would be ripe again for merchandising, new non-shitty movies and TV shows.
....but nooo.... some thick-headed suit in the boardroom of DC/WB just has a raging hard-on for Harley Quinn (probably because they watched too much Corey Chase HQ cosplay) and they won't let this SS shit fucking die in a fire where it belongs.