I think the joke is the stains are so big that they seem like they're part of the outfit.
I'm hungry...stainsGood work, Sanae...Ah, Reimu-senpai!
Thanks for all your hard work as well!Exterminating youkai is fun, but...Ohhh!
I've got these sweat stains...Reimu-senpai...She exterminates youkai so violently, yet her armpits stay so clean...