Disclaimer: I own nothing. The original idea of why Lulu wore such a dress stems from the fanfic Seven Wonders by LadyMercy. I just expanded the idea.
Lulu was a serious, responsible woman. She worked hard for the people she cared for and the things she cared about. Anyone could see it in just the way she kept Yuna safe and how she watched out over the boyish Wakka.
But the thing she worked the hardest in constantly was that of her black magic skills.
In fact, she had a very unique way of constantly practicing these set of skills; that which required the use of her dress. The very dress many of which scoffed at her for for its impracticality in many aspects.
She practiced the magical aspects aligned with using Water and Blizzard spells by working to keep herself cool in her daily life in Besaid and much of the rest of Spira, which was often at a high temperature or humidity in and of itself. The furs of her dress being much too hot for such weather.
Where areas the aspects aligned for Fire spells she used to keep her warm in snowy areas, the low cut of the dress and the skirts not being able to keep the cold from hitting her, even with the dress's long sleeves.
The belts were to make sure she could keep herself silent so as not to attract fiends with the clacking of the buckles being much too loud if not kept in check by Lulu's prowess.
The low cut of the dress forced the Black Mage to use her abilities to keep the dress from showing what shouldn't be shown to the public...
Lulu was a woman who wasn't prone to making mistakes and if she did she corrected them, thus the use of her magic so laboriously throughout even her daily life. One slip up with her magic in a battle could result in hurting her companions, her friends instead of her intended target and that was a mistake she was not willing to make.
Not even once.
And so Lulu continued to practice in this manner, through both of her pilgrimages before Yuna and even on Yuna's own pilgrimage. Even her teammates eyed her dress curiously from time to time, but Lulu would never explain, she didn't feel she had to.
She was just practicing after all, even if when or if they found out just why she wore such a dress they would think she were showing off.
Lulu decided it was just the price to pay to keep her magic in check and to not slack off.