Although Lucy is a lot like a angel, she still has her flaws. In the movies, she does get jealous of Susan, but she also fails to stand up for seeing Aslan to her siblings (I might be wrong on that one). In the books, she does have some flaws in PC and VDT (they're mostly the same with the movies, though).
For one thing, she may be great in some things, but not with everything else. Peter was more fit for being the High King because he was older, more kinglike, and, well, magnificent.
But Lucy didn't mind, and besides Peter being in a higher seat than them, all of the Pevensies took equal jobs in ruling Narnia. Peter and Edmund do struggle with issues sometimes, in both the movies and the books, but they eventually see their mistakes and grow from them, just as Lucy does.
And I don't think being quietly observant is what brought Lucy to do all those things. Her childlike innocence and faith were what got her to find Narnia first in LWW and see Aslan first in PC. She was sent out to find Aslan, because of her faith in him. And because Coriakin warned her before, she was able to stop Edmund and Caspian from doing harm to each other (I liked it better in the book, though).
So, because of her amazingly strong faith in Aslan and desire to follow him, she became the character with the most heart in the series. Of course, she's not perfect, and she usually realizes this, but she'll do what she can to do what's right at all times. I wouldn't call her a guardian angel, but I'd call her a light and inspiration among the Pevensies and many other Narnians.
After all, in the end, she's not the real saviour, but Aslan is.