Say rather, Harry is too often softened by his emotions. He chooses softer approaches, is gentler, kinder... warmer. Not what Mab wants in her Winter Knight.
True. Mab definitely wants Harry to be less compassionate. (Incidentally, I think this is a mistake. His compassion is often what fuels the heights of power that he reaches, pushing him to attempt things that he would otherwise not consider. I can't help but think that a less compassionate Harry would also be a
weaker one. I also think that Mab, by her nature, is incapable of recognizing this, though.)
The problem is, Mab's actions are not likely to make Harry less compassionate, or less warm, or less inclined to help people simply because they need it. The only thing they do make Harry "less" of is less inclined to agree with anything Mab says on general principle.
What
is making Harry less compassionate, on the other hand, is the Winter Knight mantle, and Harry's need to use it. This would seem to imply that if Mab wants him to be more Winter-like, she could best do so by putting Harry in situations where Harry would be willing to draw upon it. Possibly in situations already designed to encourage people to be less compassionate: where no matter how hard you try you're losing people, where you need to have "bucketloads of sangfroid" to get through it--you know, like the descriptions we get in Ghost Days and Skin Game of what fighting the war against the Fomor to protect Chicago and other cities is like. Which, in fact, Harry would be glad to do because he would be defending people, so Mab wouldn't have to go through her usual rigamarole of having to force Harry into doing it. Which he has plenty of time to do, even, because Mab doesn't seem to need him more than a few days out of the year. Which would help him gain and maintain allies that he could use to help fulfill Mab's purposes when needed (and he would: see Cold Days).