You should notice that the tradesmen aspire to be greater than they are. This is apparent in their choice of play, their attempts to use words and language far above their level, and their misunderstanding of the audience. It is this desire to be greater that provides the humor in the play, not only for you - the audience - but for the royals in the play itself and for the fairies. In contrast, the aristocrats in the play are simply themselves. This is reflected in the beauty of their language and ways of interacting with others.
One way that Shakespeare makes the distinction between tradesmen and aristocrats is with their language. While the tradesmen tend to use bawdy, course language (especially bottom), and misuse words, the aristocratic lovers and fairies speak in poetry. Another differentiating feature was likely the costuming used. Shakespeare describes the ass head to be used in the play, and the other tradesmen costumes would have been as coarse. In contrast the lovers and fairies would be vested in finer clothing.