Contemporary viewers in 2015 tend to look down their nose at the miniskirt uniforms and dismiss them as demeaning and sexist, without taking the context into consideration. Remember that in the '60s, the miniskirt was seen by women as liberating and empowering, as it challenged old mores in a male-dominated social system that had always forced them to cover up from head to toe.
And if you look into comments of the Trek actresses who wore them at that time, it's clear that they didn't see anything wrong with them at all. Grace Lee Whitney wanted that kind of uniform, which was fashionable from a real-life standpoint, instead of having to wear something similar to what the men were wearing. Similarly, Nichelle Nichols has expressed something to the effect of "You wear them in real life, so why shouldn't you wear them on TV?"
Kor
And if you look into comments of the Trek actresses who wore them at that time, it's clear that they didn't see anything wrong with them at all. Grace Lee Whitney wanted that kind of uniform, which was fashionable from a real-life standpoint, instead of having to wear something similar to what the men were wearing. Similarly, Nichelle Nichols has expressed something to the effect of "You wear them in real life, so why shouldn't you wear them on TV?"
Kor