I don't think the game looked great, but I doubt very much that that was the main reason it flopped; as far as AAA games go both nuXComs look pretty crappy too, and they were both smash-hits.
Bluntly
Grunker I think you're delusional to argue this wasn't a massive cock-up on the part of the marketing department. It was perhaps unlikely to sell 15 million copies regardless, but as I keep returning to: I like Marvel, I like card games, I like nuXCom, I ended up really liking this game once I played it, and yet, based on what was shown in the lead-up to the release I had zero interest in this game and zero intention of purchasing it. That's fucking insane, and making the argument that it was a niche product made with a AAA budget in no way explains that.
Now I'm not saying this was the game's only launch flaw. The social sim, the nakedly cynical monetization scheme, and the bizarre decision to largely focus on fairly niche and non MCU-headlining characters were all unforced errors that undoubtedly lowered the title's commercial ceiling. But I'm convinced the marketing effort was what made this game DOA.