I would say -- as a native speaker of English, but not of Russian -- that хер is roughly equivalent to words like "wang" or "dong" -- it's not as strongly obscene as "c*ck", but at the same time, it's definitely not a cute euphemism that a Russian mother would use when telling her 5-year-old son to "wash yourself down there" while in the bathtub.

So it's "moderately vulgar," and transliterating the name of Miss Hermione Granger as Хермиона would sound to Russians sort of like "Dicktoria" (as a joke version of "Victoria"). And spelling Mr. Harry Potter's name as Харри could also be phonetically suggestive of a "penis joke" in Russian, although perhaps not quite as hilarious!!! as with Hermione's name.

(By the way, when I was a very little boy, my mom told me to "wash your dork", but other English-speaking moms might prefer to say "wash your peter" or "wash your wiener" or "wash your pee-pee", etc. And the polite Russian word, by the way, is член, which literally means "member.")