While "Dream Song 14" certainly doesn't qualify as a nature poem, the natural world does play an important role in it. In fact, the connection between man and nature becomes one of the poem's central themes. Let's take a look at what Berryman is after with his "flashing" skies and "yearning" seas.
Without the nature imagery, the speaker's sense of boredom in this poem would seem less far-reaching and all-consuming.
Berryman uses nature imagery to balance out the speaker's incessant (and, let's admit it: kind of unreadable) griping.