OK, a bit more explanation.

The miko vestments are not a yukata, although the white bit is quite a lot like one. A yukata is a casual kimono, essentially. A lot of young women wear them to go to summer festivals or firework displays, and they're also common at hot spring baths. Think something like a long dressing gown.

A kimono is more formal. What's more, a woman wearing a kimono should look roughly cylindrical; no visible curves. Padded underwear is used to account for the fact that women aren't, actually, that shape.

Now, because the yukata was originally underwear, specifically for (I believe) during a bath, you didn't wear anything underneath. Japanese people still say that it's traditional, or proper, to wear nothing underneath a yukata. These days, most people do wear some, however, and I have no idea how you'd get accurate data on how many don't without getting slapped... ;)