Given the talk of Merlin and Mab I had always assumed she was Morgan le Fey - the enchantress from Arthurian legend.
She was Merlin's apprentice, as was Mab.
If she ascended to the mantle of the Winter Lady, she could arguably have been called "pimply" by Corb - maybe just another way of calling her childish and youthful. Alternatively, Harry mentions warts being a byproduct of magic in an earlier era (as well as milk turning). Maybe she had some kind of spot or blemish as a changeling or mortal wizard due to her use of magic.
If she was alive then, perhaps she followed a similar path to Maragart le Fey, grew distant from her ostensible allies in the White Council (or a prototypical version of it), leading to Merlin distancing himself from her, as Ebenezar has to Maragaret and now Harry.