Robert Frost's "Design" illuminates the speaker's reflection on a spider's web. As a side note, given it is proper to denote the narrator of the poem as being the one to attribute feelings to, one must refer to the speaker's attitude and not Frost's.
In essence, the speaker questions the design of all things in the world--the good and evil, death and life. While the poem fails to bring true closure to the questions posed (Why was the flower white?; What brought the moth to the spider?; and Is the design of everything important?). While none of the questions come close to being answered, the most important question is the poem itself.
Given that a poem, especially one like Frost's (which has planned stanzas and rhyme), is part of a design, what is the importance of the design of the poem? Is it to answer questions, or is it to make the reader think about things they may not have thought about?
Regardless, the speaker's attitude about natural design seems to be one which is both confused (given the unanswered question) and one which is certain (given the metered breath of the speaker).
Since one cannot ask Frost how he feels about natural design, one can only go off of what his speaker/narrator states (or fails to state). Even then, the question lies open to each individual reader and his or her own interpretation of the text.