Why do you think it is so grotesque?
Why? The book isn't really, even thought the movie is very faithfull to the novel. I mean, in the book, Oskar is even a bit kinder and ironic about everything.
Schlondorf's movie is, on the other hand, very foccused on being disgusting most of the time. Why? Is he trying to make you feel sick about this Germany he is portraying? I don't know.