He acts at first like a coward and a scaredy-cat because that is what hobbits do. They like to just sit around all day and be lazy and do nothing. In the middle and end of the book he actions lean him more towards fierce and brave.
Yes, Bilbo begins the story as a cowardly hobbit who dislikes adventure, but he grows throughout the story to become courageous and persevering as he finds himself content with adventuring.
