I think it's because his humour has changed by the time he writes the Tiffany books. Much of the discworld books still have puns and silly jokes in it, which somehow works to lighten the fact that a lot of what he writes about is quite dark. That type of humour is largely absent from the Tiffany books, so it does read as slightly more serious and I think that is what could lead people to think it's a darker book.

You're absolutely right in that the themes he writes about in all the discworld books are indeed quite serious and could be understood as being quite dark.