I hold my judgement on Kate until she has produced a few Male offspring.
You can't judge her at this stage, she is still work in progress.
The same way you can't measire an ex race horse until it has produced a few quality offspring.
So yes she is unfair to judge Kate at this stage.
No-one-except those closest to her-can know what she is really like. Unfortunately,part of the remit of being a Royal, is to present a particular 'persona'-a bland one-because it is 'safe'. It's just her 'public face'...not the real woman. You'd think Mantel was savvy enough to realise that.