What is the theme of sonnet 21?

Shakespeare's Sonnet 21 appears to mock the exaggerations which were common in love sonnets of his time. Its theme is very similar to that of his Sonnet 130. The poet says his loved one is as fair as anyone who has lived - but is of human beauty only: not that of the phenomena unrealistically evoked by other poets.

His message is summarised in the sonnet's concluding couplet, which may be rendered in modern English as follows:

Let them hype up, who love a story's telling:

I'll make no pitch on things which need no selling.

Read more on this and his other sonnets in Shakespeare: a Hidden Life Sung in a Hidden Song (see link below).