The emoji of a smiling cat coincides semantically with the regular human's ":)". It means joy, good mood, and laughter, because your interlocutor's joke is so funny.
It was Japan that started the anthropomorphic cats fever. They've got a whole fetish on that, because cats are such cuties! Actually the cat smiles existed long before the appearance of emojis: :3, =^_^=. Kawai!