You learn how easily swayed these commoners really are. Flavius and Marullus chastise these people for partying in the streets while they wait for Caesar to return, the same Caesar who just defeated Pompey. Flavious and Marullus point out that it wasn't that long ago that these same people cheered Pompey as he crossed the streets. In one of the greatest invented hyperboles ever, the commoners are told to go cry in the Tiber until it overflows its banks, a direct accusation of how they should be mourning the death of their beloved former leader, Pompey, not celebrating the return of Caesar.
This idea of herd mentality will play itself out completely in Act III when Brutus makes the horrible mistake of allowing Antony to speak to the crowd last.
The second minor theme that is established, in part, during this scene is the idea of how opposition to Caesar is treated. Flavious and Marullus "pull scarves" off of Caesar's images, essentially they will deface any public image of Caesar. These actions later lead to their exile from Rome.