Is sonnet 18, known for the line "Shall I compare thee to a summer's day," insincere and sentimental?

Shakespeare's Sonnet 18 could be interpreted as insincere and sentimental, but it was more likely intended to be a spontaneous creation to amuse and flatter the person to whom it was addressed. Shakespeare is showing he has the talent to create a flattering tribute based on any subject. The poet and person the poem is about might be enjoying an outing on a summer's day and talking about poetry. Shakespeare might have been challenged to compose a sonnet about his companion. This is something the metaphysical poets were fond of doing. The personification of death as a being who is proud of his accomplishment is strongly reminiscent of a famous sonnet by John Donne (1572-1631):

Death, be not proud, though some have called thee

Mighty and dreadful, for thou art not so.

It isn't much different from asking riddles. How is your loved one like a summer's day? The poet wishes to impress his loved one with his versatility, something it could be argued he might be attempting in many of his love sonnets. At the same time, the poet may be expressing his real feelings in a sort of disguise. The concluding lines of the sonnet seem sincere enough, and they express an idea which is to be found in many of Shakespeare's other sonnets—that his words have the power to make his loved one immortal.

But thy eternal summer shall not fade,
Nor lose possession of that fair thou ow'st,
Nor shall death brag thou wander'st in his shade,
When in eternal lines to time thou grow'st,
So long as men can breathe, or eyes can see,
So long lives this, and this gives life to thee.