Only that isn't true, because he's now a lead character in a major motion picture. It would be like saying Zod isn't one of the biggest Superman villains of all time because the comics almost never use him, well yeah sure, but he's also the main villain of one of the biggest piece of Superman media there is, and then he was in Man of Steel. It doesn't matter if Deadshot was once obscure, because now he isn't even kind of obscure, and now is when they're putting him in a video game. You can't be obscure anymore when you're the lead character in a huge movie. Like I said before, Negasonic Teenage Warhead, once an obscure character, but if Capcom were to put her in a new upcoming Marvel game, or a Marvel VS game now, it would be pretty weird to call her obscure because it would be happening post Deadpool.
Iron Man had a show nobody watched, I don't even know what your point is there. Although people knew who Iron Man was before, he's in some much loved video games, they just didn't really give all that much of a shit about Iron Man stories.
Some people that played MvC3 weren't even born when that Spider-Man cartoon ended, it's not surprising that everyone doesn't know every character. That isn't exactly the same as a character being revealed shortly after the big movie he's in is released, and said game comes out the following year. I've got some little nieces and nephews, pretty sure none of the, have ever looked at a comic, and they knew who Dr. Strange was. I don't think I ever really saw anyone saying they didn't know who he was when UMvC3 came out, I saw it with Rocket Raccoon, but not Strange. He's even already in a ending in the first version of the game.