Is a poem about nature beautiful because of its form, or is it beautiful because
A great question! There may be a middle ground to the answer. Beautiful natural objects, and beautiful poetic objects, might both be considered beautiful because of complex or harmonious formal properties that evoke certain responses (this is, roughly, Kant). If this is the case, the a beautiful poem about something ugly would function differently from a beautiful poem about something beautiful. In the former case, the beauty would be purely formal; in the latter, it would be in part representational.
Again, a great question, although I suspect it might also be misleading. Many well-known poems about nature are not actually about nature in a straightforward sense. Poems are rarely like landscape paintings. (Come to think of it, neither are landscape paintings.)