A cynical part of me says: Most people who donate money do it because they like feeling better about themselves; making the world the best place possible is a secondary concern at best. Without an in-depth look at her giving habits, this suggests she is purchasing those same fuzzies from writing books outside the shadow of the behemoth that is Harry Potter.
A second part of me, concerned with the fairness of the situation, says: It seems odd to criticise JK Rowling for not maximising the amount of money she donates to charity when she's donated far more, both in relative and in absolute terms, than most people.
The rest of me does not necessarily endorse either of these positions but refuses to step up and say anything.
While, in both relative and in absolute terms, she has donated more than most people, she is also lucky enough to be in the unique position to be able to. I doubt it's just to make her feel better, because she was homeless at one point, and often empathy is grown out of experience. However, while she may not see it this way, I feel that, in a way, this is depriving these charities of money, because if she rode her fame, she'd make more, and therefore be able to donate more. In a recession (aka, a depression but we don't want panic), that's even more important than usual.