That actually depends a lot on adaptations in recent years. For example, it may be hard to believe now, but Iron Man was once considered too obscure of a Marvel character to make a successful movie out of. He was a big name in the comics, but that didn't translate into being marketable for general audiences. Of course, all detractors were proven wrong and thanks to the MCU he's ascended to Spider-Man levels of fame.
An even bigger example is the Guardians of the Galaxy. They were obscure even to Marvel fans. I remember when Rocket Raccoon was announced for UMvC3 (three years before the movie came out) and people were like "Who the hell is this guy?" or "Wow, a talking raccoon? That's just stupid". Then the movie came out and everyone knows about the Guardians, and thinks Rocket is one of the coolest and most original concepts they've had.
I guess Iron Fist is popular enough, but he's still not that big of a name outside of comic books fans. While most people probably know about Daredevil without really looking into it much (as is the case with many comic book characters without a blockbuster adaptation), I'm not sure if the same could be said for Iron Fist. Still, I hope he makes it in and isn't cut just to make room for an A-lister. I liked how MvC3 went the extra mile to include obscure names on both sides, when it would've been easy just to use all the biggest names and nothing else.