Shakespeare's "Sonnet 130" has often been called something along the lines of a "winsome trifle" designed to be merely funny, which it is, but Shakespeare is also gently satirizing the conventions of the Petrarchan--as well as his own--sonnets in which the beauty of the subject is elevated to heights beyond the achievable. The imagery of the sonnet centers on the beauty, or the lack thereof, of Shakespeare's mistress of the day, and the images are polar opposites of the conventional images of beauty we would expect to see in a sonnet. Where we should see beauty, we see wires; where we should smell sweetness, we are repelled by reeking breath.
Aside from her brown skin (which should be "milky white") and wires for hair (which should be "smoothly-flowing tresses"), Shakespeare's mistress's cheeks demonstrate a color not found in nature:
I have seen roses damasked, red and white,/But no such roses see I in her cheeks.
The choice of rose imagery is especially clever here because red and white roses that are "damasked" are variegated, that is, they are streaked with red and white. In addition to pointing up his mistress's lack of color, Shakespeare has chosen an image that would remind readers of the Wars of the Roses, a series of battles between the houses of Lancaster, whose symbol was the red rose, and York, whose symbol was the white rose. The damasked rose, both red and white, is emblematic of the resolution of the war between Lancaster and York. Given Shakespeare's acute awareness and use of symbols throughout his works, it is unlikely that he would overlook the chance to tie this humorous image to painful history.
Shakespeare has, in effect, created a very funny anti-sonnet: a sonnet in form, 14 lines with alternating end rhymes, with a closing couplet, all in iambic pentameter, but whose imagery mocks the conventions of both the Italian and English sonnet.