"Not your ideal expectation of yourself" is probably not enough, but if it's causing the nail-biter significant harm, then potentially, yes. If you've got someone who's tried unsuccessfully to stop biting their nails, they do it as a stress relief mechanism, it's causing significant bleeding/infection, they're hiding their hands out of embarrassment, they're having problems at work because it's grossing people out, etc, that could probably qualify as a behavioral addiction.