While I'm not excusing the actual dress sphere character designs, as I understood the in-game fluff for the Dress Spheres, they were modelled/taken from actual people (hence why the Singer Dress Sphere held fragments of Lenne), so Yuna didn't intentionally dress as a scantily clad gunner, just that the Gunner Dress Sphere was taken from a gunner who believed in bullet proof nudity.
That said, Yuna did become more significantly easy going in the two years between X and X-2 (not having the weight of the world on your shoulders will do that), judging from the Eternal Calm video.
Never mind Red Chocobos, I dreaded facing a party of Yellow Chocobos in the prequel bit since that usually resulted in a TPK.