Peter Piper Picked a Peck of Pickled Peaches

Yeah the editors don’t like Ikematsu chapters lol

I think she's wearing "haramaki pantsu" (belly warmer underwear; try googling 腹巻 パンツ for pics). Hers is a functional one with a cute pocket (add ãƒã‚±ãƒƒãƒˆ to that search query to see pocket ones and some that look like hers). I feel like you usually see old men wearing haramaki in anime, not sure what the cultural implication/stereotype is but my best guess is that it's dorky and outdated and that the belly warmer, along with her childish underwear, are meant to show that her fashion sense never really matured--presumably because she's a virgin and does nothing but draw hentai.