I suppose mini-skirts are impractical if you're climbing trees and something, especially if you have stockings underneath (pretty much guaranteed they'll get torn)... otherwise, they're very comfortable and easy to move around in. As for sitting in them, it all depends on your own and society's attitudes over revealing parts of one's body. Some societies demand for a woman to cover almost everything, while in others it's normal to walk around with very little covering (look at the clothing of some African tribes, for instance). It's the one-piece costumes like Seven's catsuit or the TNG season 1 pyjama uniforms that would be really impractical. You'd have to take them off every time you go to the toilet! Also, clothes that are too tight and cover the whole body can be very uncomfortable, but that depends largely on the material: whether it's soft or crude, and whether it's too warm in it in high temperatures. Kira's uniform is a Bajoran Militia uniform. How is that a "one-off"? Just because there's no other prominent Bajoran command officer in the main cast?