LeFey seems to be an epithet. The name may be reflective of her power (mostly in the form of her influence) but I don't think it's a Mantle as such. She got the name after she was what she was, not as part of becoming it.
As for the favor to Leah, maybe? If Leah knew Harry might be a Starborn, and a powerful wizard in his own right, that kind of hold may have been enough for her to agree to it. Though Jim did say that she made a bad bargain, where she could have gotten a lot more if she wasn't rushed, which seems to be indicative of some kind of payment.
The most relevant thing seems to be her personality. She chafed against the chains of the Laws, how they restricted her (and other wizards) from doing the right thing, etc... Most of the Mantles we've seen appear to have both power and restrictions or responsibilities. Fae, Warden, Gatekeeper, etc... I don't see Maggie being willing to let herself get chained down for whatever that would entail (Which we can't say without knowing what Mantle she might have) rather than leaving herself the option to do whatever she wants/deems right.