Shakespeare presents the commoners in a very unflattering light in Julius Caesar, Act 1, Scene 1. How does he criticize them through the words and...

The commoners are portrayed as selfish and fickle.

In the first scene, Shakespeare portrays the commoners as self-centered because they are just happy that there is a holiday and do not care why.  Marullus and Flavius object to the fact that Caesar defeated Pompey, the former consul of Rome, in a civil war.  They consider Pompey’s death and Caesar’s rise to power as an insult to the Roman people.  The commoners shifted their loyalty easily from Pompey to Caesar.

This scene does provide comic relief, as the commoners respond with puns to Marullus and Flavius’s questions.  The two men are Patricians, of Rome’s upper class.  They seem to be affiliated in some way with the conspirators, or at least the conspirators know them.  The Plebeian (lower class) laborers do not care why there is a holiday.  They are just glad they do not have to work.

Marullus launches into a tirade, chastising the commoners for not being more respectful to Pompey’s memory.

Wherefore rejoice? What conquest brings he home?
What tributaries follow him to Rome,
To grace in captive bonds his chariot-wheels?
You blocks, you stones, you worse than senseless things!
O you hard hearts, you cruel men of Rome,
Knew you not Pompey?  (Act 1, Scene 1)

Marullus and Flavius do not support the way Caesar came to power.  They do not approve of decorating his statues.  Marullus questions whether they should remove the ornaments from the statues because it is the Feast of Lupercal, but Flavius does not care.

It is no matter; let no images
Be hung with Caesar's trophies. I'll about,
And drive away the vulgar from the streets:
So do you too, where you perceive them thick.
These growing feathers pluck'd from Caesar's wing
Will make him fly an ordinary pitch,
Who else would soar above the view of men
And keep us all in servile fearfulness. (Act 1, Scene 1)

This is actually why they are punished later.  Casca says that they are “put to silence” for removing the ornaments, but nothing specific (Act 1, Scene 2).    It is additional ammunition for Cassius and the other conspirators in their plans to assassinate Caesar, and also helps them recruit Brutus to their side.  Caesar is abusing his power.