Yes, it is a metaphor referring to the cold, dead body of John Keats, the great English poet who died of tuberculosis in Italy when he was only twenty-five years old. Shelley is saying that our tears cannot bring Keats back to life. His body is as cold as if it were covered with frost. Since Shelley is writing a pastoral elegy, the image of a body covered with frost is appropriate as well as striking. It corresponds to the later line "He is made one with nature." Shelly does not imagine him being buried but lying out in the open and gradually being absorbed into nature. There is a literal truth to the statement that we should weep for the poet. His death was a great loss to English literature. He fully intended to fill many volumes with his poetry, as he says in his sonnet beginning with the words "When I have fears that I may cease to be," and we can only imagine what those volumes might have contained. This tribute to Keats as Adonais is full of so many beautiful lines that it is sufficient by itself to show that Shelley was likewise one of literature's greatest geniuses. Robert Browning had such a high regard for him that he was astonished to overhear a man in a bookshop telling someone he had actually met Shelley at one time. Browning wrote a little poem titled "Memorabilia" to commemorate the incident. The opening lines are:
Ah, did you once see Shelley plain,
And did he stop and speak to you
And did you speak to him again?
How strange it seems and new!