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.
If I guy said that I would probably feel pretty embarrassed, so maybe it depends on the girl?
lol. I love your name. you got it from watching the office didn't you? and to answer your question, I think you should just leave it alone. I make a great effort to keep my undies covered up. but some girls like to show off their thongs and stuff. it isn't wronge to comment. but you might get yelled at.
in these days when considering the fashion trend its not that unusual to see some butt clevage and yeah the girls underware as well is often visible , myself I might notice the odd cute butt now and then but I wouldn't ever feel comfortable saying anything to the girl about this . think its too risky an area and any comment is going to be viewed as perverted no matter what you say , it be like a girl commented on anything below our belt as they say , its just a no go . unless you have a really close relationship with the girl and know she's ok with it but definity don't say anything to a new girl you just meet
When I was younger and people walked around with their underwear showing, they were automatically given an extreme wedgie! Needless to say, very few people walked about with any portion their underwear in view. Think of it as vigilante justice.