No, not her fault, I wonder if she volunteered to be Harry's godmother or did Mab order it as part of the bargain with Margaret to give birth to a star child in the first place?
My theory is the White Council wanted Margaret to give birth to one, but because of her relationship with them and opinion of them, she didn't.. But when she changed her mind to have one, she went to Mab to get protection for the kid, thus the bargain. A bargain which gave the baby protection, but also guaranteed when it was all grown up that Mab would have a star child as a Winter Knight.. I don't think Harry ever really had a choice, he was maneuvered into thinking he had one, but in reality at some point Mab would have informed him of the bargain his mother made.
My own WAG is that the WCouncil was too afraid of the "Starborn as Destroyer" (albeit some individual WCouncil members may have demurred, and taken independent action); but the BCouncil thought that (Starborn/Destroyer, either way) it sounded like
exactly their kind of "shortcut to power," so they wanted it.
I think Margaret was onboard at first. I think she backed out of the Raith relationship when she realized how Papa treated his sons (i.e. killing them), but I don't know why she turned away from the larger "BCouncil" plan ... maybe she figured the Whamps had too much influence, and she couldn't have a child under BCouncil auspices that could be free of Raith's "kill all sons" schtick?
I'm pretty sure Mab was
ALL about getting herself a Starborn WK, absolutely
loved the idea! I suspect she was instrumental in "getting Margaret to see the error of her ways" (with respect to Raith, and the BCouncil) and instructed Lea to put herself in LeFay's path often enough (but not in a substantially adversarial way) that when she was on the run, pregnant, and knowing Raith would launch an Entropy Curse as soon as he could)...
Margaret LeFay absolutely
grabbed at the chance to make a "Faerie Godmother" bargain with Lea!
I entertain the notion -- as a WAG within a WAG -- that Mab actually
planted the idea of a Starbabe, so that she could harvest herself a SB WK when the child was ripe.
And no, I do
not think that kind of long-range planning -- and manipulation of mortals -- is beyond Mab's ability.