Captain Marvel was a little boy who could become a superhero by screaming "Shazam!" It was unapologetic wish fulfillment directed at young readers. Still, Fawcett Comics was worried that dumber children were having trouble relating to the character, so they created Captain Marvel Jr., a little boy who transformed into a little boy, perfect for readers with less elastic imaginations. Then they added the Lieutenant Marvels, three hillbilly Shazams to make their illiterate readers happy. Finally, they came up with the idea of a rabbit version -- Hoppy the Marvel Bunny -- for readers who were shitty assholes. Oh, I almost forgot -- after all that, they came up with this weird idea of making a girl one. Handicapped kids got their own Barbie before girls got their own Shazam.
The '40s shouldn't have expected much from a teenage girl who, by my count, was a seventh-rate Shazam knockoff, but what they got fell short of all expectations. Mary Marvel had three working brain cells, and two of them died trying to figure out a sanitary napkin. Her enemies were usually cranky teenagers with no powers who routinely eluded her by throwing something in her eye and walking away while she fussed.