The worry for Mab is not that Winter is changing both Harry and Molly for the worse, but what if it is a two way street? That they are also changing Winter, and that is what worries Mab.
Harry got Mab to ante up compensation effectively using the rules of Winter against her, not how things usually go between Knight and Queen. Then Molly in The Good People finds an unexpected loophole in the rules and exploits it to the hilt, the Winter Lady can be Lady Bountiful during Christmas and the Mantle permits it, fully.
The humanity of Harry and Molly are making things uncertain for Mab, no wonder she orders Harry to kill Molly, it would solve the humanity issues of both in a single go making things more predictable. The problem is both performed spectacularly against the Fomor and the Titan, not despite their humanity, but because of it. Even the Redcap, The most loyal, vicious member of the Winter Court has nothing but praise for Molly and he can’t lie.
If Molly becomes Queen, she may not becomes the clone of Mab envisaged, if Winter is changed she might be quite different and retain much more of her self.