The 'index of its kind of a funny story' might also imply a categorization within the story's own unique framework. Suppose the story is about a group of misfit animals. The index could be a guide to each animal's funny traits and their respective story arcs. It could show which animal is the most accident - prone and where in the story all their clumsy moments are detailed.
The humor might come from the way the story is told. If it has a lot of vivid descriptions of funny expressions or sounds. Like in a story where a parrot imitates a person's sneeze in a very exaggerated way. It makes the story more engaging and funny.
The unexpected twists usually make it funny. For example, if a character is supposed to be very serious all the time but suddenly does something completely silly like dancing in the rain without any reason. That kind of deviation from the norm can be a great source of humor.
A story can be 'kind of funny' when there is an unexpected twist. For example, if you expect someone to be really good at something but they end up being completely hopeless at it. Like a professional athlete who can't ride a bike.
They often have a great sense of absurdity. They can create situations that are completely out of the ordinary but still seem believable within the context of the story. For instance, a character might suddenly start talking to animals in a very normal way as if it's an everyday occurrence.