If anything, this is a poem about frustration, disappointment, and irritation. It is about an “author” who is annoyed that her friends took her book to get published without her consent, and it is also about the difficulties of revising said book (the speaker suggests that she is unable to make her book any good). More than anything else, then, this poem’s sounds reflect, imitate, mimic, etc. the various experiences of frustration that the speaker describes.
Take the poem’s first lines as an example. The lines start out sounding normal enough—“Thou ill-formed offspring of my feeble brain, / Who after birth did’st by my side remain”—but then as the speaker contemplates what her friends did by stealing her book the sentence becomes cluttered with extra clauses: “Till snatcht from thence by friends, less wise than true, / Who thee abroad exposed to public view, / Made thee in rags, halting to th' press to trudge, / Where errors were not lessened (all may judge).” All these extra phrases—sounds—“less wise than true,” “all may judge,” etc. frustrate the flow of the lines, and imitate the speaker’s irritation.
Elsewhere, the poem reflects the sounds of revision and writing, with all their frustrations: “I stretcht thy joints to make thee even feet, / Yet still thou run'st more hobbling than is meet.” Every line in this poem, you could say, is “stretcht” to make it be five beats (pentameter). This line mimics the sound of the stretching—notice the assonance, all those long E sounds at the end (“thee even feet”). Longer vowels take longer to pronounce, giving the line the effect of being longer or more “stretched.” This “longer” sound also imitates the speaker’s own perception of writing and revision as a kind of prolonged torture (“stretcht” can’t help but remind us of the Rack, a medieval torture device).
In fact, if you take a glance at the poem, you will notice that long vowels are all over place. They are less pleasant than their shorter counterparts, and they give the poem a slightly more melancholic or doleful cast, especially when the word is a word like “feeble” (1) or “poor” (23). This reinforces the speaker’s own frustration, mirroring the content—just on a sonic level.