'Seems confident' isn't really not saying much because Jonathan Strange is the sort of the man to be confident about many things, including that giving up cards, drinks, and the company of other women is sufficient inducement to persuade a curate's daughter to marry him. If confidence was enough to impress Arabella, she wouldn't have waited for him to get an occupation.

Also, Jonathan telling her not to worry is naturally only going to make her worry more! Arabella spent most of the time he was away worrying about him, he certainly can't tell her to stop now. Strange is probably lucky that he's a bit more cleaned up for the ball, or his mad hobo appearance would have set off endless worrying. There is something about his appearance that she can't help commenting on--

"Not when it comes to dressing yourself, I'm afraid," she'll tease gently, lightly running her fingers along the gaudy embroidery of his coat and smiling. "This color doesn't really flatter you."