Understanding your book would be hard enough, let alone understanding mature jokes in the book. As you said, it all depends on the tone though I think to ease the tense situation in the book sometimes it is nice to add something immature. Something which could put a smile on someone's face.
Like: 'his skin was tingling, though his face felt hot and oddly numb; his back ached as the taut muscles tensed further, and every nerve felt as if it was burning' but when his friend tapped on his shoulder and held his hand tightly, looking at him giving him a little smile, it all seemed to be calmed.
Ok I am not a good writer though I can imagine the situation in my head! Hahaa.
If something's HARD to do, then its NOT worth doing!