Mason is obviously a talented back. We saw that last year. He's not an elite talent like Gurley but there is nothing wrong with RBs that are simply above average. Glad to have him on board. He's now healthy again and, unsurprisingly, looking good. Let's enjoy him while we can. Not only are we spoiled to have a #2 RB this good but once his contract is up I am sure he will leave in an attempt to land a starting gig elsewhere. As he should. He isn't even splitting carries with Gurley (understandable, since Gurley is one of those rare true bell cows). He could at least get split carries elsewhere.
I don't think he fumbles that much. All RBs will fumble occasionally. Gurley actually had 2 or 3 drops in one game alone vs Arizona if I recall. I do with Mason would block better. To his credit, he seems to have improved that aspect of his game a bit.