Believe it or not you are not actually obliged to write a review so if you're not happy about the amount of words you have to write - and let's be honest they aren't that many - then maybe you should not write a review? The reason Amazon require a minimum is because the whole point of the review is to inform the Amazon community and to ensure that the reviewer is actually bothering to put some thought into the review; afte rall any imbecile can write a one word review which will provide no enlightenment whatsoever.
Sadly this has passed you by and in consequence we get a rather pointless and nonsensical review in which you state the book is amusing - a good start for a book which is intended to be humorous - but then you go on to state it is not your style: wouldn't that indicate that for you it wasn't amusing? So we now have a book that is amusing - which is its purpose - but which you have given only one star becasue it is not the humour you want it to be. An amusing book which you recognise as being amusing but not the right sort of amusing? Eh???!!!
It's almost as if you are saying that you did find it funny but because some of the subject matter is rude you feel guilty about having found it funny so the book must be punished.
The fact that you mention that there are too many references to body parts and bodily functions leads me to believe that you know nothing about the humour of Ricky Gervais, Stephen Merchant or Karl Pilkington. This would be the equivalent of you buying an Autobiography by Roger Federer and then complaining about the fact that there are too many references to tennis.
For what it's worth my advice to you is that if you're struggling to write 20 words then you are not really able to write a worthwhile review so it's a really good idea not to bother because you will not do the product - or yourself - justice.