Shakespeare's narrator says his mistress doesn't have snow-white skin, perfume for breath, etc. What he's saying is true--no one has skin as white as snow, or breath like perfume, or the ability to float above the ground. Therefore, he's not insulting her. What, then, is this poem about

what novel are you talking about?

remember, the goal of an adjective clause is to add more information to a noun or a pronoun. as you can see from the examples above, you can add information by including a longer adjective clause or tighten up a sentence by turning the adjective clause into an adjective phrase.