The graphic medium does mean a certain attenuation of atmosphere. In Lovecraft's text, we are taken into Dyer's mind, and he admits that he is a slightly unreliable narrator, partly unhinged by the horrors he has witnessed. As a drawn character, even as the narrator, he becomes someone who we readers watch along with the other members of the expedition (and the monsters); he may still be the central character, but his perspective is no longer as privileged as it is in the original text, and that's probably unavoidable. (Dave Sim, for all his many faults, actually had some great moments in Cerebus where we could appreciate the points of view of particular characters, but I think that needs a different kind of story-telling than is really possible here.)
Anyway, a must-have for anyone who is even a mild Lovecraft fan, or indeed for anyone who hasn't yet tried him but is wondering what the fuss is about.