Is Yoiko hentai?
No
Rated R+ (Mild Nudity) on MyAnimeList and not tagged as Hentai.

TV · 1998 · 20 episodes · R+ - Mild Nudity
Comedy, Ecchi, School, Seinen
Her name is Esumi Fuuka. Standing tall at 163 cm with a slender bodyline, she must be a university undergraduate; however, in reality, she is only an elementary school student. With a body figure of an adult and the mentality of a small child, she creates many misunderstandings around her.
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