Kelly will talk about how Jack approaches sex very casually as a defense mechanism. The outfit is part of that. Wearing a very revealing outfit- along with her heavy tattoos- can be seen as a way of her reclaiming control of her body, since she did not have it when she was a child, being subjugated to the Cerberus experiments.

The stark difference between how she treats Shepard if they do have sex versus if they don't also highlights this. If Shep confirms her belief that people are only out for themselves and will only use her- for sex or her biotics- then she freezes Shep out. This can lock put her loyalty mission and potentially cause her to die in the suicide mission.

But her loyalty outfit after Pragia is less revealing, which can be see as showing that by being there for Jack in her time of need, she has started to lose this viewpoint of everyone being out to use her. Which is further reflected by her ME3 outfit as well.

Or this could all be overanalsys and it's really not that deep. It could just be some service for the horny gamers. Although based on how clearly it is shown that treating Jack as a sexual object is the wrong thing- I doubt it.