It is a dialogue said by a boy who watched and criticized a theater play. I also translated it myself for checking if I correctly understand the context.
わるいけどお先にしっけいするぜ。 --> It is not good but I have to say goodbye.
こんな茶番劇見てるなら --> If I have to watch this foolish play...
反省房へとじこもってへそのごみでもとってたほうがよっぽどましだよ --> I think secluding myself in repentance room and doing/taking..... would be much better.
I'm not sure if it means doing/taking unimportant thing.