So wait, is your argument that since D&D has zombies, it should have even more gore and mature content? Because that's a silly argument, if that's what you're angling for. It would be just as silly as saying that since so few examples of art go beyond G rating, that all the art should be kid-safe.
It has had gore (Corpse-Collector comes to mind as something that wasn't in the BOVD, it's roughly PG-13 gore) and mature content to certain degrees, and you can't argue that it's always been G, because it hasn't. Most of the art has more accurately been PG, and all products should be treated as being at least PG, with the occasional allowance for more.
The zombie example and the Walking Dead comparison, is an example that any attempt to make it G, would make something that has lost most of it's substance.
Taking into account a non-movie based version of ratings. I'll directly quote the DC Comics ratings system, and titles published by DC Comics themselves:
E – EVERYONE
Appropriate for readers of all ages. May contain cartoon violence and/or some comic mischief.
Young Justice, and many the adaptions of DC animated series.
T – TEEN
Appropriate for readers age 12 and older. May contain mild violence, language and/or suggestive themes.
Superman, Batman (Batman Inc had a scene in a slaughterhouse), Wonder Woman (has some blood in the death of mythological creatures), Demon Knights, and most DC Comics titles.
T+ - TEEN PLUS
Appropriate for readers age 16 and older. May contain moderate violence, mild profanity, graphic imagery and/or suggestive themes.
Animal Man (has a man being eaten in half), Swamp Thing (has a chainsaw attack), Batwoman, I Vampire.
M – MATURE
Appropriate for readers age 18 and older. May contain intense violence, extensive profanity, nudity, sexual themes and other content suitable only for older readers.
Hellblazer, Fables and practically everything published under the Vertigo Comics label.
Using the above examples, D&D has generally been around a T rating, some books go to about T+ like BoVD or the occasional gruesome monster/scene that's existed in a couple instances in 3e.