You can add Jewel to the list for sure, but her quality of poetry in no way diminishes or adds to the validity of Leonard Cohen's work. I think there is a tendency to discount the poet that happens to be a musician or the musician that happens to be a poet, which is silly because the two often coexist so beautifully. One of my favorite poets is David Berman, he's a great song writer too, you may be familiar with his band "the Silver Jews."

I haven't read Jewel's poetry, but my guess is that it's pretty bad. I have read some of Leonard Cohen's poetry and as you would expect, given his songwriting, it's good. My guess (and this is just a guess) is that many of the poets of "our time" that you would put on the list of those worthy of being called "great" would also consider Cohen's contribution to the field of poetry/lyricism "great." So while your annoyance that Jewel is a widely known "poet" may be justified, I think you should be grateful that Cohen is helping to keep the appreciation for poetry alive. Also, I would wager that fans of Cohen are likely aware of other modern poets whereas fans of Jewel... well, probably not.