People have the freedom to read what they want without fear of ridicule. Libraries have a responsibility to serve their patron base by building collections relevant to their interests, making them available, and promoting them.
If there are books intended for a younger audience that have mass appeal to adults, they should be available to adults. If those titles are in a YA section where adults are not allowed to enter, perhaps acquisitions staff should consider adding a copy of those titles to the adult collection. If budgets or space don't allow that, readers' advisory materials should point patrons to those titles. Harry Potter, Twilight, and The Hunger Games are all series that my library has added to the adult collection.
Graphic novels are a format rather than a genre, but they are treated as such despite the diversity of genre and style. Many people think of Sunday comics and superheroes and assume that all graphic novels are for children. I think that the focus on how graphic novels impact reluctant readers contributes to this belief, too. It never fails to amuse me that people think graphic novels lack literary merit; many of them are quite complex works of art.
Since so many graphic novels have been adapted into popular films and television shows, libraries should be collecting them, yet many do not, believing that their patron base has no interest. How do they know that? Perhaps there is a whole patron base that stopped coming to their libraries because they never found what they were interested in reading. Additionally, graphic novel collections should be shelved according to their intended audience.