Which part of the metamorphosis might make the story a comedy

Gregor's attempts to crawl on the walls

Explanation:

When Gregor begins to accept his new identity he starts crawling on the floor, walls and ceiling. His sister, Grete, decides to remove some of the furniture so as to allow Gregor to have more space for his new pastime. This action of the main character crawling on walls like a bug might be found as a comedy in the story.