User blog comment:Omega natsu2/Episode 177: The Eclipse Plan, Review./@comment-1359322-20140412080742/@comment-7418318-20140412085652

In the manga she wore a black body-suit. I don't think that would have been any harder to animate than the dress, but to be honest, it makes more sense for Mira to wear the dress, since they are still running through the city.

Standing out even before entering the palace grounds is something an infiltration team should avoid and that dress is certainly less conspicous than the body-suit. But I do hope she'll use transformation magic or something to switch to the body suit once they are inside, since it's better suited for battle.