In Shakespeare's Sonnet 130, he is listing the attributes of the woman he loves, but not in a necessarily positive light. He speaks of her looks (her eyes, her lips, etc.) and points out what they are not.
My mistress' eyes are nothing like the sun;
Coral is far more red than her lips' red...
The speaker goes on to note that her hair is like wire, her cheeks are not lovely and her breath "reeks." Though he loves to hear her talk, music is much more appealing than the sound of her voice:
If hairs be wires, black wires grow on her head.
...no such roses see I in her cheeks;
And in some perfumes is there more delight
Than in the breath that from my mistress reeks.
I love to hear her speak, yet well I know
That music hath a far more pleasing sound...
In listening to these first two and a half quatrains, we might think that the speaker's "mistress" will not be pleased with his poetic efforts: his relationship may be over before she reaches the end of her sonnet. However, the last two lines of the third quatrain introduce an observation that puts all of what he has written before, into its proper context: how can he or anyone else compare his love to a "goddess," when no one has ever seen one? In essence, he might well be saying that she is a goddess as far as anyone else truly knows.
This brings us to the second part of your question—the kind of poem Shakespeare is ridiculing: he is making fun of love poems that make impossible comparisons with creatures they know nothing about. If goddesses do exist, a woman cannot logically be compared to one because no man has ever seen one in order to make a fair and valid comparison in the first place.
Shakespeare ends his sonnet by praising his lover on his own terms, not based on false allusions:
This is what Shakespeare means by “false compare”—unjust comparisons that not only ignore the possibility that the woman may be beautiful in her own right, but also miss the value of the beloved in the eyes of her lover...
Shakespeare finds that everything about his mistress is lovely to him and "rare." Rather than using empty praise to describe this woman, he uses logic instead which is objective rather than subjective. His descriptions are in no way meant to discredit her, but simply to put the process of praise in perspective. Comparing a woman to an unknown quantity is as effective and "honest" as comparing her hair to wire or saying her breath "reeks." An unsubstantiated comparison is meaningless to the speaker.