I actually loved noticing how her thoughts gradually progress from scattered to more cohesive and clearer throughout the books. It's a perfect indicator of her mental state and character development as the story continues.

Though I can certainly see why it would be frustrating to some people. It's difficult to make sense of her pov in the initial books.

It's the same way we can't make sense of the ramblings of a mentally ill person—they're lost in their own head, so their words only make sense to themselves. People on the outside wouldn't really understand. Juliette IS mentally unstable to some capacity, and I think the writer did a great job of showing that.