The secondary characters are all the commoners who end up putting on the play at the end. Given that the metatheatre there is the only interesting thing about this (one of Shakespeare's worst plays), those characters are very important. Also, think about what they're saying about the common people, juxtaposing them against the nobles like that and having them be the butt of every joke. Shakespeare wrote most of his plays for the absolute lowest common denominator, the groundlings, so what does it mean that he's making asses of them (literally) here?