Final answer:
Marullus is upset because the citizens celebrate Caesar's victory over Pompey, demonstrating their fickle loyalty and quickness to forget their former heroes.
Step-by-step explanation:
Marullus is bothered by the citizens because they are celebrating Julius Caesar's triumph over Pompey, who was once celebrated by these same citizens. Marullus sees this as a betrayal and a lack of loyalty, as well as a sign of fickleness in the public's opinion. This displays how quickly the people can forget their former heroes and change their allegiance, which deeply upsets Marullus.