That would also make him Satan...so I think you're missing the real issue there.
A terrible misconception.
Satan is little more than a title which literally means "the adversary". What you meant by Satan was Lucifer, whom is the king of Hell. Mephistopheles was among the first fallen angels to accompany Lucifer to Hell. Mephistopheles was among the highest demons including Beelzebub, Belial, Azazel, and others.
Mephistopheles is a tragic demon; he seems more trapped in his position rather than evil and terrible.