The assertion that "Spider-Man is the most popular superhero," is partially accurate, though it hinges on exactly what metric one uses. No one can deny that Spider-Man is one of the world’s most beloved and by no means obscure superhero figures, with a consistently high ranking across a number of categories, including, but not limited to, the following: the sales of superhero merchandise; the recognition factor of said superhero among both children and adults; and levels of social media engagement, where Spidey seems to actually excel, apparently, over characters who are popular even in the comic book medium, like Iron Man.
Sales of comic books show that Batman often beats Spider-Man, which indicates a strong amount of competition between the two characters. When we look at the sales figures for merchandise or at the appearances of either hero in public places, we almost always see Spider-Man ranking at the very top, with Batman not very far behind. In claims of this being some sort of objective Superhero Popularity Contest, supporters of the idea that Spider-Man is the most popular superhero tend to point at him being on top in these conversations.