Be honest with your mother. There's no cure for stuttering, make sure she knows that. If you're comfortable with stuttering around her, just tell her that. Tell her there's nothing wrong with stuttering, because there isn't. It's your choice if you choose to work on it or not, if you're choosing not to, tell her she has to accept that. You stutter, working out's not going to change that, it may improve your life a bit, but if you're fine with how your life is, it's not for her to judge.