The irony of Casca's description is that it shows he has a total contempt for the common people. He loses no opportunity to denigrate them as repulsive, mindless, and easily swayed:

If the tag-rag people did not clap him and hiss him according as he pleased and displeased them, as they use to do the players in the theatre, I am no true man.

He even remarks that they are dirty and have "stinking breath." The irony in this is that as a conspirator against Caesar, he will have to rely on the support of the common people to make sure that their cause prevails after Caesar is killed. Indeed, he is the first of the conspirators to urge Brutus to speak to the common people and explain their actions after the assassination is carried out successfully (Act III, Scene 1).