What is the true subject matter of Shakespeare’s “My mistress’ eyes are nothing like the sun”?

This is a great sonnet to look at, because in it,
Shakespeare deliberately pokes fun at other poets at the time who greatly exaggerated
the qualities of the women they wrote about, producing idealised damsels that were in
every sense of the word, angelic and too beautiful for this world. This poem, on the
other hand, deliberately and with great relish demolishes such approaches, by presenting
the mistress that is the subject of the poem with a grittly
realism:

And
in some perfumes there is more delight

Than in the breath
that from my mistress
reeks.

Thus Shakespeare is
deliberately playing with our expectations as readers as we come to yet another love
poem that challenges what we expect. However, the point of this poem is that, in spite
of the "imperfections" of the mistress, or indeed because of them, the speaker of the
poem can love such a woman just as much as he scorns poetic cliches of
beauty:

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

As any she belied
by false compare,

Thus this
excellent sonnet is really poking fun at other poets and the idealised and unrealistic
way in which they described their beloved. By taking a more realitic tone, the speaker
thus shows his true love for the mistress.