Not that it matters, clearly, but I've been doing Yoga for nearly 30 years. In that time I have worn all sorts of clothing, including beachwear, leotards, nighties, PJs (not joking) and skirts, short and long, as have various teachers I've had over the years.
In my most body-conscious years I vastly preferred hiding under a wide flowing skirt than clingy leggings. Haven't ever done the tights/leggings under a skirt though as I get too hot and I prefer for certain poses to put my bare foot on a bare thigh or calf. Leggings don't move enough for that that's why I often wear flowing Yoga pants.
I did gymnastics at school for eight years so pretzeling my body into poses while wearing just knickers (alas, these days they are sadly of the sturdier kind) was never an issue during my body-confident years.
And anyone busty who does Yoga regularly knows it's not the skirt you worry about in upside down poses but what happens with what you wear up top. My favourite lose t-shirt, loose trouser combo is definitely not suitable for mixed-sex classes, that's for sure.