(The irony is that it is actually, really, really hard to explain what Merrill does in words, but it isn't a difficult concept. If you look at the poem in the post below you will get it immediately.)
I think this is really, really nice and subtle. It isn't kitchy. There are poems written in the shape of swans, and poems written entirely in spirals. In my opinion less successful. But "Christmas Tree" reminds us that part of what we love about every poem is the text of it. Poems aren't one dimensional--they have sound, and content, and then they have text. The form the poem takes on the page is what the poem "looks" like.
Where the line breaks are placed is often related to the content of the lines, and just as often related to the sound of the poem--or the meter--but I think it is rare that a poet thinks quite so deeply about the text as a body, as a casing for the insides of the poem--and then makes that body reflect the insides of the poem as Merrill has here.