While well, and happy, and properly attended to, she had great good humour and excellent spirits; but any indisposition sunk her completely. She had no resources for solitude; and inheriting a considerable share of the Elliot self-importance, was very prone to add to every other distress that of fancying herself neglected and ill-used.

There does seem to be some implication of that while she felt good (physically) also note that it is qualified by the words "happy" and "properly attended to." As long as she is already in a good mood, as long as someone is giving her attention things are fine. However Mary doesn't do well by herself, she had "no resources for solitude" so she felt ignored. Also it sounds to me like "any indisposition" would ruin her day. "Oh darn! I got a hang nail! How Low I Feel. Darn Charles has gone off and left me again, the kids are loud..." and it spirals out, so on ad nauseum.