This is a book of two halves, really. The first half (and a bit more) is rather standard mockery of cliches, as the three Witches (not my favourites among Pratchett's recurring characters) experience the delights of foreign travel and people who don't speak your language.
But when they get to their destination, the city of Genua, towards the end of the book, things really take off; it is as if the Brothers Grimm hit New Orleans - and who is that woman who looks like Granny Weatherwax? A lot of Pratchett's writing is about Story, in a way, no doubt reflecting the amount of time he has spent thinking about narrative in the last few decades, but I don't remember any of the novels (after the first couple) highlighting it quite like this.
So, harmless enough at the beginning, much more serious at the end.