Even though it's a graphic novel, the drawings I've seen are not particularly graphic. The librarian said they are designed to reveal to reluctant readers, although "readers" might also be a bit of a stretch, since the cartoons I've seen don't have words.
However, does the subject matter give the mother the right to keep the book? She refuses to return it (it's the only copy the library owns) so they can evaluate it. She also says that if the library replaces it, "I'll have somebody else check it out and I'll keep that one."
She also says she plans to burn the book.
When asked what bad outcomes the book might lead to, she wrote, "All different kinds of things."
Good grief! Of all the possible things to get het up about, this is one? When I was a kid we used to tell dead baby jokes, another example of jarring juxtaposition. And we turned out okay.