Why do you think Shakespeare starts the play with a bunch of commoners joking around him?

This is to show how fickle the public is. Julius Caesar opens with the tribunes of the people chastising the plebeians for being fickle. They refer to the masses as "You blocks, you stones, you worse than senseless things!"(1.1.34). This imagery of the masses as stones will continue throughout the play. They are in fact a fickle group of people, easily swayed by whoever is speaking to them, as evidenced later in the play when Antony turns a hostile crowd into a mob against Brutus and Cassius.