Yes, actually. Sam's coming from a very rural kind of society -- births, deaths, and other things are probably less taboo as subjects than they are in other times and places. In traditional folklore there are also a number of stories about graves being robbed for bones for various reasons. Part of this of course arises from the simple problem of space. We know nothing of hobbit burial customs, but Tolkien was coming from a long established country which had been using the same churchyards for centuries. In digging new graves, old ones would certainly be discovered now and then, and skeletons would be a not entirely unfamiliar sight. And of course hungry hobbits would know the various ways that additional calories might be extracted from the bones of more prosaic origin, like chicken soup.

Trolls aren't exactly polite society anyway, and do try to eat other humanoids -- Sam knows Bilbo's story -- and a kick in the rear is a typical humorous method of revenge. Yeah, I could see Sam telling it.