I see. But I don't see how Teresa fit as a Mary Sue. Despite her superiority, she still mocked humans and toyed with them. She can be even cruel to them like when she kicked Clare in the face without a second thought. Things started to get even more interesting when her human side was resurfacing. Her sudden death. All those makes her a really interesting character but YMMV.
You've asked why I'm not a fan of Theresa. I dislike Mary Sue type of characters and Theresa was such a character. She was perfect in almost everything and reading stories about such perfect beings is not really interesting. Theresa started to be a little interesting when she started to loose her perfection, she became more human; and died soon after that.