Injustice constantly straddles the line between grim and goofy. One moment, Superman will use his laser vision to murder Shazam, and the next moment, a Batman impostor will just blow up various members of the Suicide Squad because they're either useless or he just plain dislikes them. (Calendar Man, one of his would be victims, gets a pass just because the tablet controlling his bomb collar decides to stop working.)
Even when the story involves a truly grim death, there's something darkly funny about it, such as Tim Drake reuniting with Batman and getting immediately killed by General Zod in the middle of their moment together. The hands involved with steering the franchise are aware that this is primarily a video game universe, so tone and logic only matter when they really need to.
WB's DCEU films have gotten a fair amount of flack for seemingly having an incredibly narrow subset of fans that the studio wishes them to appeal to, one that seems to be predominantly male. But while Injustice may be a franchise built around costumed dudes punching each other into submission, the women are very much active players in the story, with their own arcs and development. Harley Quinn gets to finally beat down on the Joker after years of him doing that to her, for example, and the resulting evolution of her character following his death allows her to bond with Black Canary over what Superman has taken from them.
Injustice 2 makes a more concentrated effort to appeal to everyone's inner child. The Gear system that allows for the fighters to all have distinct looks. Sure, the Gear is tied to stats and is important, but don't be fooled, you're playing dress up with comic book characters like you're a kid again. Combined with guest characters like Hellboy and the Ninja Turtles, it's clear Netherrealm wishes to draw in anyone with an interest in DC's library of characters. And if that doesn't do it, the victory poses that look like lenticular comic covers will.
The best part of Injustice's success is that it seems to have happened in part by accident. Sure, the game was destined to be a fun time, but it probably wasn't expected to be one of the most fun and audacious DC-branded franchises in the world today. Netherrealm's superhero fighting game isn't just well liked, it's one of the best fighting game franchises around, supported by a strong intersection of fans that happen to be nerds, fighting game fans, or both. And by having its roster encompass not just the more popular names, but also those in movies and TV shows, plus some more lesser known characters, Netherrealm has gotten its bases covered to be one of the most wide reaching DC media franchises out there.
As the DCEU attempts to move forward on a better foot, it could do a lot worse than to look at what Injustice is doing and copy off of that playbook. For all the fighting franchise's faults, it's doing a lot of things right -- certainly more things than the DCEU seems to be able to.