Kara's incredible Super-Skirt
The thing is that it is a short skirt. A very short skirt. As drawn by Micheal Turner and Ian Churchill it barely covers her bottom anyway, and it is a loose pleated design. And yet it remains modestly glued to as much of her ass as it ever covers even when her cape is cheerfully being blown upwards.
Now I know there is a long tradition of superhero underwear defying physics, generally in the whole area of not getting destroyed. They should build tanks out of the material Bruce Banner uses for pants - it might turn them purple but this would be a small price to pay for something that could withstand the direct blast of an atom bomb.
But I digress. The difference is that not only are these panties are intended to be seen, but they are often clearly visible to people around her. Any cheerleader can tell you that there's a difference between what you wear while doing cartwheels in a short skirt in front of 100,000 people and underwear that you don't plan to share with an audience. Silver age Supergirl was flashing her panties all the time - particularly after she started trying out different costume designs in the 70's.
They were usually blue, if you're interested.
When it was just Turner and Churchill it might be a personal choice of the artist, but now Supergirl is all over the DC universe and being drawn by many different artists. They may not be able to agree on what age she is supposed to be, but the skirt remains stapled to her thighs, so I can only assume it's an editorial decision.
So the question is if the sight of a bikini bottom is so terrifying, why permit this costume design in the first place? Why have this absurd situation where the art has to be carefully arranged so reader can't see the super thong even though characters in the comics are constantly getting an eyeful? Why not give her some shorts like Stargirl? Why not give her any other damn costume design but one where you have to keep coyly hiding bits of it that are clearly visible to anyone behind/below her?