Coding a carddav export / import would be easy. But realistically every platform will need to just use whatever format Mastodon uses.

And then there’s the question of what would importing fedi contacts into an address book app actually do - most contacts apps would merge contacts based on email address or phone number, neither of which are available in a federated situation. If the app merged based on name that wouldn’t work great either because few people use their full/real name on fedi.