The average moviegoer could not give less of a shit about cartoons or video games. Claiming that these characters were in any way known in the mainstream because of them is just absurd.
RDJ was a washed up has been in the eyes of the moviegoing public, Hemsworth was nobody and the characters they were playing (particularly Thor, but Iron Man too) were unknown. Marvel still sold it.
The only two members of the Avengers who were in any way known to the mainstream were Captain America, who was mainly seen as a joke, and Hulk, who had flopped in the cinema before. Marvel essentially managed to build a colossus out of the bits of their universe that no one else wanted.
Iron Man was known to the general public before the movie. It's one of the reasons the movie opened to such a high gross it's first weekend.