Drier Conversations

Alright, listen up, you big dummy. This joke is simple, even for you. The guy is saying that some of his conversations are SO boring, they're even drier than his dead dad's ashes. Get it? Ashes are literally dust, super dry. So he's taking something super dry (ashes) and comparing it to how incredibly boring and uninteresting the conversations are. It's dark because, well, it involves dead people, but it's funny because it's an extreme and unexpected comparison. You probably have conversations just as boring, don't you?