When we think of the word mood, we associate it with the emotion produced in a literary text through the choice of words and content. Of course, a mood can change as a text progresses and so often there may be two or more moods in a given text of literature. When we think about this excellent sonnet, therefore, it is clear that the mood of the first fourteen lines is mocking and humorous, as Shakespeare deliberately plays with the conventions of sonnets and presents his beloved in a less than attractive light with such descriptions as follows:
And in some perfumes is there more delight
Than in the breath that from my mistress reeks...
This is hardly what Shakespeare's readers would have expected from a love sonnet, and so the mocking tone is established. However, it is in the rhyming couplet that closes the poem where there is a distinct shift in mood. The last two lines almost have a defiance and seriousness about them that are in sharp contrast to the rest of the poem:
And yet, by Heaven, I think my love as rare
As any she belied with false compare.
These two lines affirm the love that the speaker has for his mistress and also underlines his dislike of exaggerated conceits. Thus there are two tones in this poem: a mocking tone in the first fourteen lines, which is replaced by a serious tone in the rhyming couplet that ends the sonnet.
Those would be the tine not the mood