Joni Mitchell would be a [D] at least, since she has two albums rated at 12 on George's scale, although her page is, as you say, very far from complete.
As for Kate, she has an album rated at 5 stars on the MP3 scale, which as far as I can tell translates into a 14 (see below). This means she'd get at worst a [B].
Rating Translation Table
|Full Scale||MP3 Scale|
šPresumably, George made the assumption (correctly as far as I can tell) that he'd never be able to give a 15 to an album being reviewed in MP3.
˛George said somewhere on his site that lesser artists, though they have fewer highs, also tend to have fewer lows. Again, I'm therefore assuming he didn't expect to encounter any "atrocious beyond all imagination" albums from artists he thought only fit to review in MP3!