Mephisto's realm is not the same Hell that is portrayed in the Bible, and that is mostly because of the Comics Code Authority (CCA). The CCA persuaded American comic book publishers to self-regulate their works and was established in response to two events: the publication of a book in 1954 by psychiatrist Fredric Wertham called Seduction of the Innocent, which vividly details how comics can negatively affect children, and the Senate Subcommittee on Juvenile Delinquency that followed. The CCA seal was voluntary, but parents were less likely to buy comics without one. Unfortunately, the rules that comic publishers had to follow were incredibly strict; some examples include "Ridicule or attack on any religious or racial group is never permissible" and "Scenes dealing with, or instruments associated with walking dead, torture, vampires and vampirism, ghouls, cannibalism, and werewolfism are prohibited."
With all these strict rules, how was Marvel able to create their own Satan in 1968? Per Polygon, Stan Lee and John Buscema envisioned Mephisto as not from this world, which made him more science fiction than supernatural and, as Lee remarked, helped establish the Silver Surfer as a messiah resisting temptation. Fortunately, the CCA in 1971 permitted comics about vampires, ghouls and werewolves if they were "handled in the classic tradition such as Frankenstein, Dracula, and other high caliber literary works" from the likes of Poe and Doyle, which explains why Marvel published the first appearance of Ghost Rider the following year.
