Can't she just summon a mud bath? It won't disappear, and even after it dries it'll keep covering her body. Or since it's more of a "spa" thing, is like the yukata being an "inn" item and thus disappearing?
Occasionally there's a rare moment in comedy where something truly memorable happens that stands out far above what you are accustomed to, and reminds you of how unfunny things are 99.999% of the time. This chapter is one of those moments.