She does get a lot of panel time, but so do a lot of other important characters. "Important" and "Main" are not the same thing. Jiraiya was an important character and had a shit load of panel time (when he was alive). Other characters are also important to the story.
You can have multiple main characters when the story is titled and focused on those characters. Lots of mangas (and American comics) have multiple main characters. In this case, Kishimoto has consistently described his manga as "...a story about a boy named Naruto..." As such, the supporting characters, important and not unimportant, serve to develop the one MC.
