Cara shared a photo of her elaborate costume on Instagram, which she completed with not one, but two lightsabers. The head-to-toe look served multiple purposes. One, as Cara shared in the caption, was that she was definitely "Watching Star Wars in style"—she could pretty much guarantee that no one else in the theater would be sporting the same outfit. Two, was that it allowed her be completely disguised—there was no one to tell whether Cara or another hardcore fan was under there.
The only potential flaw in the outfit depends on what the visibility behind the mask is like—it may be perfect for getting into the theater unnoticed (or, like, noticed for different reasons), it does not seem ideal for viewing purposes. Especially if she saw it in 3D.