"Appearances" is a major theme in Sonnet 130, since our speaker spends a lot of the poem talking about what's wrong with his mistress's looks. He does a pretty complete dissection of her face, her body, and her smell. He doesn't say anything at all about her personality, but instead sticks to his laundry list of problems with her appearance. This gives Shakespeare a chance to poke fun at our obsession with looks and to show how ridiculous it is to ask any person to live up to some ideal of perfect beauty.
By pushing his criticisms of his mistress's appearance to the edge, the speaker makes his return to the theme of love even more effective. The contrast between mockery and love is what drives the poem.
Even though the speaker eventually says how much he loves her, he has said such nasty things about his mistress that it makes him hard to believe. He has damaged his credibility.