With the exception of The Beyonder comic, there is actually no logical reason for Mephisto to get involved in Peter’s life. The only reason it happened is Quesada wanted to end the marriage. There are a lot of other means to have ended the marriage then have a character who belongs in Dr. Strange, Ghost Rider or The Avengers NOT ASM. Basically the entire concept as well as the execution and aftermath has been a colossal failure and the biggest blight on the nearly 60 year history of Spider-Man.