This sonnet is about loving someone in spite of their shortcomings. Bill spends the majority of the poem talking about his loves flaws. He compares her lips to coral rather than something lovely like a rose or a ripe apple. He says that her eyes are not bright like the sun. He compare her hair to wire. He out and out says that her breath stinks!, "Than in the breath that from my mistress reeks." These are not compliments. Then, in the end, he turns the poem on its ear. He says, "And yet, by heaven, I think my love as rare/As any she belied with false compare." The most important phrase in this poem is the "and yet." He loves her even though she has flaws. True love exists in spite of the flaws of your mate. I think this is the point of the poem. She has all of these things wrong with her, but he loves her just the same.
In the sixteenth century, a form of poetry called the blazon was briefly popular. “Blazon” is a technical term usually used to describe heraldry. It always involved a detailed summary of all of the main features and colors of an illustration and also described the position and relation of one picture to another. This method of depiction was translated into poetry and was used to portray the features of the human, usually female, body. A typical blazon would start with the hair and work downward, focusing on eyes, ears, lips, neck, breasts and so on. Sometimes, it would start at the feet and work its way up. (One famous example of the blazon is English poet Edmund Spenser’s description of Belphoebe in book two of his poem The Faerie Queene.) This form was well suited to the style of courtly love poetry that was flourishing at this time, as it allowed writers to project an idea of an idealized and distant woman whose features they could admire from afar.
Shakespeare’s “Sonnet 130” is interesting because it works by inverting the traditions of the blazon form and the conventions of Petrarchan love poetry which idealized the description of the female body.
All the twelve lines do not praise or idealize the beauty of the physical features of his lover, but on the contrary criticize her physical features by revealing the shortcomings in them by contrasting her physical features with their respective idealised poetic versions:
Her eyes are not as bright as the sun, her lips are not as red as the coral, her complexion is not fair as snow and her hair is not blonde. Her cheeks are not like roses, her breath is not sweet smelling and her voice is not musical and pleasant to hear. She is certainly not a heavenly beauty, there is not divine or angelic about her. BUT still she is unique and the poet values her for her uniqueness by concluding that there is none like her.
Shakespeare is criticising the Petrachan modes of praising and glorifying the beauty of the female form by a love struck youth. These poetic terms by repeated use had lost their freshness of appeal and had become mere cliches. Hence Shakespeare's sonnet 130.