Since One More Day, there have been many complaints on the internet that the satanic supervillain Mephisto, wouldn't be interested in undoing the marriage and that he wouldn't have the power to change history so that Peter & Mary Jane were never married.
The Lee/ Buscema Silver Surfer stories established the villain as obscenely powerful, with the ability to traverse the universe instantaneously. He claims that he could kill the Silver Surfer in a heartbeat, but is more interested in the man's soul. He generates treasure beyond imagining from nothing and can create living monsters. He even dissolves the barrier Galactus had constructed to keep the Silver Surfer on Earth.
Since the dawn of time, seldom have I sensed such goodness of soul‑ such purity of spirit‑ as I sense within the Silver Surfer. All that you are, all that you stand for, is abhorrent to the lord of the lower depths! So long as you exist, Mephisto's scheme shall ever be in jeopardy.
One complaint about the story was the lack of set-up, which was atypical for JMS. While Mephisto's appearance was predicted by many on the internet, because he works in these types of stories, there were no shocking last‑minute revelations tying Straczynski’s final issue to earlier storylines. It wasn’t suddenly revealed that Mephisto had been manipulating events throughout earlier issues, when he could have had a role in earlier stories. He could have given a prisoner the book which later provided the Shade with his powers in JMS's second big Amazing Spider-Man story. Mephisto didn’t have a sudden war with the Great Weaver or cause the storm that brought Peter, Mary Jane, Doctor Doom and Captain America to the same airport, just in time for Peter and Mary’s Jane reunion. The only revelation in OMD Part 4 tying to previous issues was the identity of the little red-haired girl, but there was no serious attempt in the previous issue to obscure that she was the possible future daughter of Peter and Mary Jane.
Featuring a supernatual/ cosmic character in a Spider‑Man story is somewhat incompatible with the idea of Spider‑Man as a down to Earth superhero, but any solution in which the marriage was erased was going to have that problem. Quesada also explained his reasons for using Mephisto over Loki, or any other mystical characters within the Marvel Universe.
Mephisto made more sense. He had the potential power and he also hasn’t been seen in a while. There was also the fact that he had dabbled in affairs of the heart before with the Silver Surfer ‑‑ and, let’s face it, this is quite simply the kind of thing the devil would do. Also, from a neophyte’s point of view, a new fan picking this up, it’s very clear who Mephisto is. A deal with Loki doesn’t strike the same chords and would tie this story into Thor mythology ‑‑ and that just complicates matters. And why would Loki honestly care? Quite simply, the story of “One More Day” revolves around one of the most classic devices in literature, the Faustian Pact.
In OMD, it's built around the classic Faustian pact. However, Mephisto is an interesting character within Marvel, I remember reading Stan's account of creating Mephisto. And while he had some of the trappings of Lucifer or Beelzebub, he is not meant to be Satan or have any religious implications. Stan built Mephisto as a super villain, but used the archetypes of the traditional iconography of the devil from classical literature and illustration. He always stopped short of making or naming him Lucifer, Satan or Mephistopheles or saying he was the devil. I get why he would create a character like this; it's low hanging fruit. The devil, or the idea of a devil, has been one of the greatest villains and mischief makers in literature for centuries. But, Stan most likely didn't want to start digging in and entrenching this super villain character that would interact inside a superhero universe within Christianity or any other religion. Also, there were probably greater sensitivities to doing this during the ‘60s than there would be later, as we created characters in the ‘70s like Daimon Hellstrom: Son of Satan – who incidentally is not Mephisto's kid. So, while some may look at a character like Mephisto and say, "Hey, he's Lucifer," I would venture to say that he is something else.
While Mephisto's role as the antagonist was controversial, things would have been quite different had the antagonist and his wife not agreed with his offer.