I have a magazine from 2010 talking about Minimates, which were a line of extra-articulated building block figures mainly based around geek culture IPs, and how great it was that Minimates was making minifigures of comic book characters like the Avengers that were way too niche to be picked up by big brands like Lego or Mega at the time.
@Shiverpeaks has a point in that before the MCU, everyone knew who the big characters were (Spiderman, Batman, Iron Man, Hulk, Thor, Superman) and the basic lore behind them and maybe even read some comic books for their favourite heroes (my grandfather read lots of Thor comics and he was a typical working man with a family). However, you are also right in that the MCU has made niche properties like Guardians of the Galaxy and Avengers that only devoted geeks would know about mainstream. We're seeing the same thing with Star Wars, where everyone knew the big OT characters, but now they're making previously niche characters/ideas like Ahsoka, Thrawn, the New Republic, and Mandalorians mainstream by taking them from their homes in niche cartoons/comics/novels and putting them in the equivalent of prime-time television.