Stop bodyshaming! She looks fine - maybe not to your taste, but who the fuck are you? Nobody. Not everybody likes the same thing - you don't like it, some people do, that doesn't mean she's 'too thin'. You could say she's 'too thin for my taste' which would be more accurate, but would mean accepting to yourself that you're pointlessly judging something that's none of your business.
Re the 6-pack question, no that's utter bullshit, you just need your belly fat to not be too much to cover your stomach muscles, and your stomach muscles to be toned/big enough to see under a thin layer of fat. My 6-pack was visible when the gym measured my body fat as 13%. But men tend to have a lower body fat percentage than women, for a man 13% might not be that low. But you wouldn't have to be 0% for a 6-pack to show, for sure. a 6-pack is a sign of health and fitness, 0% fat is a sign you might be dead.