....I do still get the impression his comments were intended as a slight set down. I think he must know Emma's views on her own ability when it comes to dancing-he has showed himself extremely familiar with her throughout and Emma does not strike me as likely to downplay any real skill she might have. Emma herself also ' felt his comments to be aimed at her' and though she might be overly imaginative (it does occasionally happen :)) I believe she is in the right this time. Perhaps her (in my mind very natural) excitement about the ball and about having a new and charming partner to dance with did rub Mr. Knightly the wrong way. I would say it did, and I think he couldn't resist bursting her bubble a little with a comment marking the unimportance of something she was excited about and knew she was good at.
In most exchanges Mr. Knightly has every advantage. In this one, I give the point to Emma. It doesn't seem as if she gets a chance at a ball often and being a wet blanket seems ungenerous. (Reading the passage, I don't see that she was attempting to inspire admiration or had the desire to quarrel with him-I read it as her being young and excited at an out of the ordinary treat There may, of course, be a difference of opinion on this. Seeing the conversation the way you do, I would have to agree that Mr. Knightly is justified in his remarks)