Like it or not, Diana Krall is generally categorized as a jazz artist and is one of the most popular and best selling as well I believe, so its hard to argue that she has nothing to do with the state of jazz.
Jazz light is still Jazz. SO what happens there does have something to do with Jazz, even if it may not be every jazz fans cup of tea.
Ella Fitzgerald sang "A Tisket A Tasket" and has sold a lot of records. Pretty light! Does that make her not a jazz artist?
Diana Krall plays piano as well. Ella just sang.
BTW, I consider Ella Fitzgerald to be one of the all time greats and my personal favorite female jazz vocalist, along with Dinah Washington (also pretty light at times) and Billy Holiday (not as light overall perhaps). I'm just pointing these things out to help balance out the argument.