The quality of being silly or trivial is frivolousness. If your mom wants to have a serious discussion about college, but you can't stop talking about your favorite reality TV shows, she might complain about your frivolousness.
Do you burst into song when your teacher asks you a serious question? Or dress in elaborate costumes that trip you up and get in the way while you're playing volleyball in gym class? Your frivolousness means you're carefree and happy — you probably spend a lot of time laughing. But it can get in the way when you really should be listening hard or getting down to work. The root of frivolousness means "broken, crumbling, or worthless."