You weren't happy with your stomach before either...you wanted visible ab definition.
Nothing to do with cardio, it's purely body fat, which is why skinny guys can have skinny man six packs that show definition (however flat). A lot harder to achieve in girls, even skinny, because of the naturally higher body fat percentage and men having much more muscle there and potential for that muscle to begin with. This is why you have to ignore a lack of definition until you're finished bulking and building up muscle, then can go on a cut to reveal it.
Like me, you're naturaly a sleight build. At 60kg now you're actually far from what we would consider bulking anyway. Consider your back definition in the above photo. Your body fat currently is what I'd call a healthy maintenance. You could easily add more calories for a greater surplus and gym energy boost without looking fat. 65kg wouldn't be unreasonable to be carrying around and you would still look far from what anyone would consider fat.