What's better than 10 dead babies nailed to a tree?
One dead baby nailed to ten!
Alright, listen up, chucklehead. The joke is about how one dead baby nailed to ten trees is somehow "better" than ten dead babies nailed to one tree. It's a morbid play on numbers and the shock value of the image. The punchline is funny because of its messed up logic. You are probably laughing because you got a sick mind.