This one goes hand-in-hand with Jeanne's dreams of being your classic American teen prom queen. To her, a white dress is the dress to wear as carnival queen, and here's why:
I had used a low-cut sarong to win the contest. But once chosen I would be a white-gowned figure out of Gone With the Wind; I would be respectable. (2.21.42)
So it's not so much that a white dress is the hot thing to wear as queen, but the opposite: it means respectability.
But why? To answer that, we've got to go back to the Catholic nuns… or more precisely, Jeanne's whole fascination with Catholic baptisms and confirmations.
This is how she views an orphan girl in the camp who gets baptized and then confirmed:
This girl had already been baptized. What I witnessed was her confirmation. She was dressed like a bride, in a white gown, white lace hood, and sheer veil, walking toward the altar, down the aisle of that converted barracks. Watching her from the pew I was pierced with envy for the position she had gained. At the same time I was filled with awe and with a startled wonder at the notion that this girl, this orphan, could become such a queen. (2.13.28)
Jeanne's completely obsessed with the idea of dressing like a bride in that white gown because it garners attention and position. But this isn't the kind of attention that bra-burners back in the 1960s got—no, Jeanne wants a kind of attention that's really pretty conventional by Western standards. She wants the kind of position that comes from being a bride, or a queen to—we have to assume—a king.
Whether that king is God or—later on—the many Caucasian guys (2.21.5) she has crushes on matters less than the fact that the white dress, the aisle, the whole ceremony has to do with being a good, respectable (White? We're not sure—what do you think?) girl.
And if you're picking up on a whole virgin-whore complex, hey—you're probably not wrong there either.