Shakespeare's Sonnet CXXX is written as what is called a blazon, which is a literary poem that catalogs the characteristics and virtues of the beloved, while at the same time it mocks the Elizabethan conventions of poetry that praise love. In addition, it satirizes the Petrarchan sonnet that compares the lover to Nature in terms that seem hyperbolic.
Literally, the speaker's lover is nothing like the women for whom sonnets and other poetic forms are usually written. Her hair does not flow in luxurious tresses; instead, it is as though"black wires grow on her head"; she has no rosy cheeks, and her breath "reeks." Nor is her voice mellifluous as "music hath a far more pleasing sound." And, when she walks she "treads on the ground." Yet, the speaker truly loves her because he feels that his love is more valuable than that of any poetic fancy because he loves her despite her flaws.
The anti-Petrarchan comparisons of the lover to various things lend humor to this sonnet. For instance, Shakespeare parodies the roses in the cheeks, eyes like sunbeams, perfumed breath, and walking on air. She has none of these or any other goddess-like attributes. Her eyes "are nothing like the sun," there is no "perfume" in her breath; it "reeks," and she "treads on the ground" rather than walks. She has no snowy white complexion or goddess-like attributes; yet, the speaker loves her dearly:
And yet, by heaven, I think my love
As rare as any she belied with false compare (ll.13-14)
Any false comparisons for the sake of poetry, Shakespeare says figuratively, are meaningless. It is her unique qualities that endear her, not flowery metaphors and similes.