I personally think the collar grab is the perfect reaction on Koh's part. It treads the fine line in showing that Koh still very much thinks of Wakaba, but still sends a message to Aoba that what she said was wrong without going overboard AND while showing restraint at the same time.
At the very least, its a much better reaction than a slap, something I must admit was my first instinct to do to Aoba when I first read this part in the manga.
As for Aoba, you do get to symphatize more with her the more you know about her relationship with Wakaba. Being a 'classic tsundere', it's practically a given in that you're supposed to have a negative opinion of her at first, but gradually start liking her character once she starts warming up slowly. XD