Why are her panties constantly showing?
Victorian style panties. In any case, surely it's not normal to have underwear exposed so carelessly,
They're not Victorian style. No one wore bloomers except nerds and babies. That's why they're in Gensokyo. More lolita style cosplayers have worn them than people from when they were a thing.
Everyone would go to Moulin Rouge to take a look at those sexy bloomers though.
they are called Pantaloons, faggot.
Bloomers are not panties. You wear panties under the bloomers.
Well what else is she supposed to do with her nose? Dummy!
Youmu-chan probably doesn't realise that bloomers are supposed to be kept hidden. She simply isn't aware of their effect on men. She doesn't know anything about sex.
When designing her and drawing her bloomers, ZUN was probably trying to show that she is a naive young girl who is so concerned with swordsmanship that she doesn't notice her own appearance.
"She's so immature you can't help liking her because of her straightforward and pure character" (Imperishable Night manual Youmu's profile)