The speaker compares his mistress to objects in nature. But, unlike many poets who compare their mistress as having qualities that equal the beauty he sees in nature, the speaker in this sonnet (130) notes how his mistress does not compare to nature. Her breasts are not snow white; they are dun (dull gray). Her cheeks are not red as roses, her lips not red like coral and her hair is like black wires. The speaker even notes his mistress's breath does not compare to perfume; that it reeks, which just means breathes forth, not as pungent as our present day concept of reeks. Still, it is not really a complement. He does note that he loves the sound of her voice although music is better. This is all to build up to the point that, while she doesn't compare to these things in nature, no mistress ever has.
He (speaker) concludes that he has "never seen a goddess go" which just means he has never seen a goddess at all. So those who would compare their mistress to a goddess are full of it because they've never seen a goddess. Finally, the speaker notes that his love for his mistress is as rare (as deep or significant) as any who has loved. "As any she belied with false compare" means that his love was as deep as any lover who misrepresented (belied) his mistress with false comparisons (comparing her to a goddess or the beauty of nature).
I guess the theme would be that beauty is in the eye of the beholder and that this particular beholder is mocking those that exaggerate their mistress' beauty, implying that he does not need to exaggerate.