Ofcourse Superman is more popular, i mean come on... Do i even need to say this ? I know Wonder Woman is about equality between men and women and She was created to go toe to toe with all super men as the super woman so Superman having the upper hand grind the gears of Wonder Woman fans, i can understand that but this is how it is. Batman is more popular than Superman... there will always be a hierarchy, lets look at them as products even if it's hard for us the fans. Or comics is heavy on the male fans side, i mean there are so much more male comics readers than there are female ones, and these boys identify with male characters and Wonder Woman is a female character. They can, i can admire her beauty and skills but never identify with her. This is probably why she's number 3 after Batman and Superman, and this is not a bad spot in men's comics world... lol Marvel doesn't even have a iconic female character which can go toe to toe with Wonder Woman (they push Captain Marvel but we will see) Do you realise how big she is ? but if we talk Marvel and DC She will be under the shadow of Superman, Batman, Iron Man, Wolverine and Spider-Man, i mean it can be said bros before admirable beautiful skillful ladies... lol These are childhood heroes and you know little boys... They are like ''girls yuck !'' lol boys stick with boys when they were little and girls stick with girls so Boys will always go for Batman, Superman before Wonder Woman. Even i wasn't caring about Wonder Woman at first, but later on she grew on me as i saw her in JL books and event books, i started to admire her, and now, i have tones crap about her, absolutes and deluxes and statues and figures and stuff.
If she keeps on having succesful films, she will probably bring in more female fans in the future then may be she can be more popular.