"Every situation has an opportunity for laughs"; some examples of how the ordinary citizens of Iran enjoyed life despite the oppressive regime were in knitting winter hoods for the soldiers and decorating the classroom for the anniversary of the revolution. This is one way they enjoyed life. They enjoyed it by making fun of what they are doing (using toilet paper as the garlands).
Satrapi adds comic relief by trying to add some makebelieve into the stories. Like other scenes in the story where Satrapi had said "140 Iranian bombers attacked baghdad today..." and thats when Marji and her father starts to laugh and did a mini celebration. These scenes are relavant to the story as a whole because it all ties together with all these different emotions and feelings. Like the scene explained before, that was happy and full of excitement, then after that, the emotions went back to being stressed and scared. It all has to come down to expressing the different feelings.