She resigned because of what she said at Questions?

No, her job was to guide the nation forward. She started by putting forward policies that were complete nonstarters including a tax break for the rich that stunk up the whole room something awful. Leaning this way or that is one thing, but getting the math completely wrong is another.

I think her performance at question shined a light on the incompetence of her administration, destroying her support and causing her to resign, yes.