Magus is the way he is supposed to be. You either like it or you don't, but he doesn't beg to be liked.
As for being better than Marle... in what aspect? You can't just say that one character is better than another overall, because they're all parts that make up a whole. Obviously, he's going to be better than her at physical attacks and offensive Magic because that's not really her strong suit. She's more of a healer than anything else.And she is the best healer you get, though Frog and Robo are also healers, they don't rise to the same heights as Marle does.
Personally, Magus (I like to name him Janus when I get him) is my favorite char, but I can't say that he is any better than any other character because each have their strengths and weaknesses. As for the term Magus as a name, I'm sure that he either A. took the name as a child when he was thrown into 600 A.D. so as not to appear silly to the mystics, or was B. given the name by the Mystics as a symbol for his great skills at Sorcery.
Being called Fiendlord in the japanese version doesn't detract from this, because it is a title given to him by his foes, and possibly even by his allies the mystics which are known as Demons in the japanese version. These Mystics/Demons are proud of their heritage and enjoy being a scourge to humans so being called a 'fiendlord' would be great honor indeed to anyone who chooses to lead them in battle. Basically meaning that they are the best of the worst without trying to be the best of the best.
Magus is a being who suffered a traumatic childhood and began down the road of darkness very early on. To expect him to be chipper and upbeat or even to provide comic relief is like expecting the sky to all of a sudden be a color that it's never been before. The fact that he changes sides and joins Crono's team, if you let him, shows that he is ready to change, accepting the fact that he can not change the past, having failed to deliver Schala safely from harm when he shows up in the Dark Ages as the Prophet. Much like the story of Piccolo or Vegita from DB/DBZ, he probably still has his dark side and is prone to brooding and keeping silently to himself.
As I said, you either like him for it or you don't, but he doesn't care either way.