Not to saying anything perverted but...?
Most Helpful Girl
If her skirt is caught in her underwear, it would probably be much appreciated if you pointed that fact out. Any girl would be embarrassed if her ass was hanging out in public for everyone to see.
But if her thong straps are sticking out at her sides, then that's intentional (and slutty) in my opinion and doesn't require your vocalization. A girl who has her underwear purposely on display wants the attention and guys to admire her 'assets'. You can feel free to go up and flirt with her, just not about her undies.