It depends on what criteria you are going to use to rate a guitarist. Some people will rate them on technical and musical ability as well as musicianship. Some people with rate them on album sales of the band they are in. Some people will rate them on how much of the material they write.
Me? I rate musicians from a technical standpoint, but also keeping in mind that not EVERYTHING you write has to be some musical juggernaught of Bach proportions.
Having said that, Mick Mars wrote some cheesy assed shit. But he also wrote some VERY catchy stuff and it all worked in the framework of Motley Crue - so for that I have to give him some credit. His sound is also very unique.
In a nutshell, I would say Mick Mars is VERY good at what he does, but as far as musicianship and technical abilities, he's pretty low on the totem pole in the grand scheme of things. Same goes for Nikki Sixx...great catchy song writer, but from a bass-player's point of view and in the realm of musicianship, he's pretty weak.
That's my 2 cents...let the flaming begin for daring to tell it like it is...