One of Shakespeare's central points in A Midsummer Night's Dream is to show the distinction between illusion and reality. He even shows that the happiness and success of life hinges upon illusion. The mechanicals' play within a play is central to Shakespeare's illusion theme because all of the mechanicals prove to have their own illusions about their abilities to perform a grand play. In addition, their performance of the play in society serves to link illusions with everyday life, to link illusions with reality. However, it's important to note that the plot of the play within the play is actually the exact antithesis of the plot within A Midsummer Night's Dream. Shakespeare's A Midsummer Night's Dream ends happily with all of the couples in the play either uniting or reuniting, but the play within the play is actually a tragedy. Pyramus and Thisbe both die by suicide. The antithetical plot of the play within the play shows us that while illusions may be antithetical to reality, the two can still be united.
During their performance of the play, the mechanicals are certainly subjected to a great deal of criticism and jesting, which they most likely can hear. For example, Hippolyta calls the moon, which Starveling represents with a lantern, a bush, and his dog, tedious and complains, "I am weary of this moon. Would he would change!" (V.i.250-51). However, when the play is done, Theseus ultimately praises their performance, saying, "And so it is, truly [a fine tragedy]; and very notably discharged" (355). And this line is the last we hear of the play within the play. Most likely, the players accepted their praise at face value and never became disillusioned about their abilities as performers. They remain clinging to their illusions, showing us that, even though antithetical, illusions and reality can unite.