The main idea in Sonnet 130 is to challenge those poets who use too much hyperbole when describing their loves.
The use of hyperbole and cliché originated with the poetry of ancient Greece and Rome. It was a convention during the Elizabethan era – and the royal court - in both literature and art.
Shakespeare is making fun of the clichés of love poetry. He satirises the concept of ideal beauty and the use of hyperbole.
His almost insulting insistence on the ordinariness of his lover - that her "eyes are nothing like the sun" and her breath "reeks" - satirises the conventional use of over the top praise. Other poets might suggest she speaks like music, her skin is snow-white, that she is a "goddess".
He is showing how unrealistic the conventional metaphors are.
There is a sense, however, that this is a more sincere love. To exaggerate would be to use "false compare" and to tell lies about his love - who is "as rare" as any of the women who have inspired such adoration.
Although she is no "goddess" he still cares for her, and in fact thinks that she is more beautiful than any of the women who are written about using unbelievable metaphors.