I disagree. If he was a bad writer, his fans wouldn't care if they finished it or not.
I think the problem here its the definition of "Bad writer". My definition of a bad writer is someone who writes badly. In other words, either they write with grammatical/punctuation problems, or, worse, they're unable to write in a way that captivates their readers. Any writer who can captivate their readers is a good writer. That includes Stephanie Meyer, as even I couldn't stop until I'd finished the fourth book, even if I hated all the characters and thought the plot was contrived. It's actually an amazing example of how you don't even have to write likable characters, as long as the writing is captivating, people will like the book. (God, I hated Edward.)
Is he a bad storyteller? No, I don't think so. The art of storytelling is being able to put together plot, character and tone to convey a story, and he's done that. He just never finished the story. That's not a crime and doesn't mean someone's a bad storyteller. It just means he knows which story to continue and which ones to let die. That actually shows maturity, rather than immaturity.
Martin is able to write compelling, captivating prose with characters people want to read about and does a fantastic job putting together a book. In fact, the only reason he hasn't finished the book is because of his fans. Every time a fan comes up to him and says "I love Game of Thrones!" he shrinks back and says "I know, I should get to the next book." The fans have been yelling at him, judging him, calling him a bad writer and voting him out of contests because he can't finish a book series that NONE OF THEM WOULD EVEN HAVE THE BALLS TO START, let alone get 1,770,000 words into it. Have you ever written a single story with over 1.5 million words? That's over 7,000 pages. That's more than most people have ever written in their lives, let alone in a cohesive story. Lord of the Rings was only 455,125 words.
This has all gotten into his head and forced him into a severe case of writer's block. Some people are more empathetic than others, and therefore are more susceptible to the influence of others. They begin to believe the criticism, and once they believe the criticism, they become the criticism.
So really, the reason Martin hasn't finished is your fault, for voting him out of this contest so early, thus reinforcing his negative image of himself.