"My mistress' eyes are nothing like the sun" is a parody of a blazon conceit, a figurative device frequently used by Elizabethan poets. A conceit in general is a witty, fanciful figurative device that extends through several lines or stanzas of verse and incorporates metaphor, simile, oxymoron, or hyperbole. A blazon is essentially a catalogue of a woman's physical attributes. So, a blazon conceit is a figurative device in which a woman's physical attributes are compared to objects or goddesses that symbolize ideal beauty such as the sun, a rose, precious stones, or Venus.
As stated above, Shakespeare's sonnet is a parody of the blazon conceit. The speaker evokes a satirical, almost playful tone when he compares his mistress to the typical objects of beauty found in love sonnets. Her eyes "are nothing like the sun." Her cheeks are not the color of "roses damaskt, red and white." Her breath, rather than smelling of perfumes that "delight," reeks. Her voice is grating, and she does not walk lightly on air like a goddess, but instead treads heavily on the ground like an ordinary woman. However, the speaker's tone shifts at line 13. Here the speaker adopts a tone of admiration, proclaiming that his love (his mistress) is "as rare / As any she belied with false compare." That is, she is as beautiful to him as all those ideal beauties to which she is compared. It is in the last two lines that we find the theme: "beauty is in the eye of the beholder."