As an uniformed government employee (non-postal ), I've had to think about this before replying. I can see the necessity of keeping the uniform - well, uniform, for lack of a better word. The "correct" manner of wearing the kilt would detailed, so there would be no deviations, no departures from the appearance of the uniform. For example, in order to wear shorts at work - I must wear a Class 3 short sleeve shirt, black socks extending 4" above the ankle, and one particular style of shoe. Any deviation from that, and I would be considered out of uniform, and at a bare minimum I would have to go home, change, and return - all on my own time.

Now, having written that - wearing a kilt (in USPS tartan or solid Postal Blue) would be a significant improvement in appearance for some of the local carriers. One guy was wearing his blue shirt with brown (isn't that a different carrier?) cargo shorts this summer. Give the carriers a more comfortable option than what they currently have, while maintaining an uniform appearance.
My ¢2 (plus ¢40 for postage).