Dylan Thomas' writing style was a bit out of fashion for his time. Modernism was the in thing back then, and people tended to chuck traditional forms and beautiful language in favor of wacky lines and everyday speech.
But Thomas, dear Thomas, marched to the beat of his own drummer. He didn't associate himself with any major literary movements, but he was often compared to the Romantics like Blake, Shelley, and Wordsworth, among others. You know, the guys who wrote about nature, love, the cosmos, emotions? They wrote poemy poems.
"Fern Hill" is no exception. It's an emotionally charged, flowery retelling of his youth that includes the moon, birds, and the sea. We would say, "Eww, gag, over-written sentimentality!" except that Thomas can pull it off because of his mastery of form and rhythm. The strict syllabic lines and varied use of sound in the poem make it great to hear aloud and make the poem a thing of beauty, rather than just a sentimental cheese fest. That's Thomas through and through.