In Marianne Moore's "Poetry" her tone is both argumentative and witty.  For she claims in the opening lines to dislike poetry, yet she argues her points through the medium of poetry. Her use of the word "fiddle" suggests, perhaps, the meaning of her use of the term "half-poets" later in stanza four. 

Moore's use of "half poets" in line 14 denotes those who force emphasis upon details in their poems; this enforced meaning and emphasis is not genuine.  It is not

elephants pushing, a wild horse taking a roll, a tireless wolf/under/ a tree, the immovable critic twitching his skin like a horse...

All these are phenomena that are worthy of poetry.  But, the "half poets" are not real poets because they overanalyze things and try to force meanings upon phenomena instead of letting the phenomena being natural and genuine.

Poetry must be the genuine outpourings of the heart and mind, not something that has menaing forced upon it.  It is not the lofty conclusions of the critics that matter so much as it is the distinct purpose of "Hands that can grasp, eyes that can dilate, hair that can rise" that make them significant. "When they become so derivative [secondary, not original] as to become unintelligible," they are trivialized.  This trivializing is what the "half-poets" do, Moore argues in her dialetic entitled "Poetry."