It's also important to remember that it's hard to judge a book objectively if you aren't a part of the target audience. I could say that a book written for 16-year old girls is cringeworthy but to them it perfectly captures what they're going through (or what they dream about). A writer who can capture the imagination of their target audience without being a part of them is definitely a good storyteller. This is actually a very difficult skill to have but one that's crucial to becoming a successful author in many genres.
By the way, the funny thing is that, as far as I remember, David also mentioned in one of his books or videos that Twilight isn't superbly written. Still doesn't matter. Sometimes beautiful prose distracts too much from the story, and I assume that's particularly common in YA and romance.
It's not that her book isn't beautiful prose. The main character is basically along for the ride and people had a hard time imagining her because she's so dull. Or maybe that's the appeal? I dunno; I didn't like the movies, either, but she got my money
To that point, the books did sell and she's got boatloads more money than I do. I often say the same thing about [insert famous pop star here with no talent]: well, maybe they suck, but they're doing better than I am, so there's something to getting out there and sucking anyway.
I try to do that more as I get older and care less about what other people think.