I don't think that it was necessary to show Carol's face. I'm sure if we could, it would be as disturbing as the face of a suicide bomber in the last few seconds before their bomb went off: A smile; bliss. The complete certainty of imminent sainthood and the sight of paradise. The face of someone who feels nothing but hate for others and is sure that God is patting her on the back and urging her on.
The reason why Willis not showing her face, IMO, is to emphasise the fact that she's become a stranger to Joyce. She's coming to realise that she doesn't know her mother; probably has never known her and only can perceive this terrible stranger breathing threats at her and those whom she loves.