The subject matter of the poem is, simply, the speaker's mistress and the speaker's love for her.  Most sonnets are written about love, but they often make use of unrealistic comparisons meant to flatter the subject and beautify the writing.  A typical sonnet, then, would never discuss a woman's lack of rosy cheeks, her bad breath, or the dullness of her skin.  This poem, and this speaker, however, are quite different.  The speaker does not draw implausible comparisons meant to flatter a lover.  In fact, he specifically says that she cannot compare favorably to the sun, coral, snow, or roses.  But this is what makes this poem special: the speaker doesn't feel the need to use lofty but ultimately meaningless comparisons, because he thinks his love is as rare and special as any in which these comparisons are used to describe the lover's beauty.  The speaker doesn't need to idealize his lover.  This leads us to the theme that true love doesn't require false comparisons; it is enough to stand on its own.

While William Shakespeare's Sonnet 130 does take a satirical turn in its treatment of the Petrarchan form as well as the traditional Petrarchan sonnet's subject matter of comparison of a lover to natural phenomena, in theme, it is not satirical.  For, while the lover's eyes are not like the sun, her hair is like "black wires," and her breath is anything like perfume, he yet loves her:

I love to hear her speak,--yet well I know

That music hath a far more pleasing sound....

Further, the speaker declares, though she merely "walks, treads on the ground,"

And, yet, by heaven, I think my love as rare

As any she belied with false compare.

The couplet, then, dismisses all the conventional comparisons that the speaker has so satirized, explicating the point--theme--of all these contrasts:  his comparisons of his lover, albeit unfavorable, is as rare for him as those beauties whom the Petrarchan sonnets laud.  For, the speaker loves his mistress just as much as the others love their beauties because beauty is subjective.

William Shakespeare’s Sonnet 130, “My mistress' eyes are nothing like the sun” is thematically an anti-Petrarchan sonnet, that satirizes the conventions of the traditional Italian sonnet by inverting the similes normally used within the Petrarchan conventions. Its subject is the beauties of his mistress, but unlike the Italianate poets who would say her lips are like coral and her breasts like snow, uses a sort of via negativa, saying that such comparisons would be false, in order to evoke in the reader’s mind the real beauties of an actual woman. The final couplet makes obvious that he really is praising rather than denigrating his mistress.