Is a poem about nature beautiful because of its form, or is it beautiful because it reminds us of the beauty inherent in nature? Philosophers tend to equate aesthetic beauty with the form of a work of art and our 'interests' get in the way of appreciating the form. However if this is the case why is there not more beautiful poems about rubbish dumps and oil spills.
If you judge a poem to be beautiful, I doubt that's because of any beauty you might find in the subject matter independently of the way it is represented in the poem. Presumably the poem is made beautiful by its "poetic qualities"--for example, its form and rythms, as well as the tropes and other stylistic features it uses to bring out subtle aspects of its subject matter, or certain broader connotative connections it might have, particularly with the reactive attitudes of its readers. (Well, that's what you get for asking a philosopher about poetry.) I don't see why there can't be a beautiful (if nevertheless disturbing) poem about an oil spill, or a not at all beautiful poem about something very beautiful. Here's one: "Oh, Taj Mahal, How you inspire me."