What does the verse metrics of "Once by the Pacific," by Robert Frost, have to do with the mood of the poem?What I mean is, is the way the poem is set up purposeful to signify other meanings?
In a skillfully crafted poem verse metrics always relates to the development of the mood of the poem. Thus, how Frost's "Once by the Pacific" is "set up" [sonnet]--which is "structure"--and the verse metrics [iambic pentameter (da DA)] are purposeful and relate to mood, while mood relates to meaning.
However, it is incorrect to say metrics or mood "signify other meanings" because metrics, mood and meaning are not equivalent terms. It is correct, on the other hand, to say metrics establishes mood and mood enhances, dramatizes and elaborates upon meaning in a poem. In other words: Meaning equates with theme and mood enhances, dramatizes, elaborates upon theme, while metrics establishes mood. To reiterate, theme (meaning) and mood are not equivalent terms; they are not the same thing, though intricately and closely related.
This clears up some of the confusion of language in your question and lays the groundwork for exploring how metrics, mood and meaning work together.
First consider that the ocean is poetically stylized as having a regular, pounding rhythm of incoming and outgoing waves (in reality the rhythm of waves is actually irregular).
 The meter of the poem, iambic pentameter [-' / -' / -' / -' / -'] mimics this regular action of the waves: in out-da DA, in out-da Da.  How the poem is "set up" (structure) as a sonnet is with three quatrains constructed of two couplets each, with one ending couplet to the fourteen line sonnet. The sonnet rhyme scheme is aabb ccdd eeff gg. This rhyme scheme provides another in-out, da DA, duple (duple: double, two) pattern in the poem. The first is the iambs (da DA), the second, the couplet rhymes (skies/eyes, if/cliff).  A duple structure can be light and gay:
- The stone
Or a duple structure can be dark and ominous, as Frost's duple structure sonnet is. Now we have arrived at mood.
The mood of the sonnet is dark, threatening and ominous: "Someone had better be prepared for rage." Frost's verse metrics (iambic pentameter) certainly establish the mood of the poem. Verse metrics is aided as well by rhyme scheme. As a result of metrics and rhyme, the mood enhances, dramatizes and elaborates upon the meaning of the poem, which is nicely revealed in the previously quoted line: "Someone had better be prepared for rage." In other words, the poetic persona foresees troubled times ahead.