In this particular line, the speaker is emphasizing his mistress's down-to-earth qualities, as he has been doing right throughout the poem. We already know that her eyes are nothing like the sun; that coral is more red than her lips; that the speaker sees no roses in her cheeks; and so on. For good measure, the speaker is of the opinion that there are some perfumes more delightful than his mistress's breath. And although he loves to hear her speak, he has to concede that music has a more pleasing sound.
And yet, despite these unflattering comparisons between his mistress and certain features of the natural world, the speaker is insistent that his love is greater than anything by which she could possibly be compared. That includes a goddess. Unlike the immortals, the speaker's mistress "treads," or walks, upon the ground. Whereas a goddess, one must presume, would have a certain elegance in her movements, the speaker's love is every inch the mortal as she treads upon the ground like any normal human being.
Once again, the speaker appears to be making a comparison to his mistress's detriment, but in actual fact he wouldn't change her for the world. She's fine just as she is, and he wouldn't want it any other way.