I disagree. Most of her lyrics (the ones I've heard, anyway) are of a confessional nature and deal with her relationships. Not exactly plumbing the depths of the human experience. But, it's pop music. It is what it is.
She has deep and thought provoking songs, which as a result can also be sad/depressing, but those are usually better. Who actually wants to hear "happy music"?
That said, it looks like you won't have to worry about her for a while:
And, when she returns to the scene she apparently will do so with a "happy" album.