I have always been suspicious of Austen-Leigh's "Memoir" which has created the image of a happy childhood for Jane Austen. There were indications that there was a tension between Jane and her mother. For instance, Jane Austen's mother was a hypochondriac and insisted on staying in the bed during Jane's final illness. Jane had to sleep on two chairs facing each other.
Here is a link to a fascinating article from the
Jane Austen Society in North America which uses Jane Austen's early works--including
Lady Susan to develop the idea that her mother was seen as an inimical figure.
http://www.jasna.org/persuasions/on-...alexander.html
What you say is true. Mothers tend to be absent, uncaring, stupid, or horrible. If they are marginal--like Mrs Rushworth in MP--then they can be relatively neutral. In
Pride and Prejudice it is an aunt--Mrs Gardiner--who functions in the role of a mother-figure to Elizabeth. And she is, in some ways, more like a beloved elder sister than a mother.