In Three Men in a Boat, what is so comical and laughable about the trout that is encased in a glass case in Wallingford that George and J visit one day?

The trout presents an opportunity for the men to enjoy some hearty laughs. This is because each guest at the riverside inn claims to have landed the trout himself. At the end of the chapter, however, both George and J discover that the joke is on them.
When George and J enter the inn, an old man is already there, smoking a long clay pipe. He claims that he caught the monstrous trout from a river and that it weighs eighteen pounds and six ounces. Next, the local carrier stops in and claims that it was he who landed the trout five years ago. The carrier maintains that the trout weighs twenty-six pounds.
The carrier leaves, and another man comes in. This new guest claims that it was he who landed the trout early one morning. After the third guest leaves, a middle-aged man comes into the inn. This time, George asks the newcomer how he caught the trout. The man is surprised but immediately proclaims that he caught the trout in half an hour. This fourth guest says that the trout weighs in at thirty-four pounds. 
Eventually, the landlord comes in, and both men tell him what the other guests said about landing the trout. All three men have a good laugh. The landlord reveals that the previous guests were Jim Bates, Joe Muggles, Mr. Jones, and old Billy Maunders. After George, J, and the landlord have a good laugh, the landlord proclaims that it was he who caught the trout. Then, the landlord tells the story of how he actually landed the trout when he was just a boy. It seems that he only used a bit of string to land the giant fish. Eventually, the landlord is called out of the room.
George then climbs up the back of a chair to get a better view of the trout. However, he loses his balance and tries to grab on to the glass case to save himself from falling. The glass case falls down, and the trout splinters into many small fragments. Both George and J discover that the trout is actually made from plaster-of-Paris. So, the last laugh is on George and J: both fell for the stories told by the landlord and his guests.