She had been wearing a very modest costume. The Goddesses who shared their abilities with her rarely covered their breasts and often made many of their appearances in the nude. They thought it was time for Mary to start wearing clothing that didn't hide so much of her beauty.
The goddesses made their recommendations. There should be an opening in her dress to leave her breasts exposed. And Mary had the idea of exposing the beauty of her pussy. The Goddess loved that idea and designed an outfit that would show her naked pussy to great advantage.