To me, the phrase 'bad writing' when appointed to a novel or something of fiction could mean a variety of things.
What do you specifically mean when you complain about bad writing? Or is it a variant of things? For example, I was with a friend a few weeks ago who told me about some novel (which I can't seem to remember the name of), and she told me it was full 'bad writing' and was difficult to finish. I asked her why, and she said it read like a teen novel, despite being aimed at older audiences. I assume she was speaking of the overall grammar or style, etc.
When I refer to bad writing, I usually mean the story as an overall, or the structure of a novel. If I read something poorly written, I'd usually look past that and dissect the story, and then make my mind up as to what didn't work.
I think more importantly, would you finish a book if it was poorly written in any respect?