Binding weight matters to the extent that it affects the feel you desire from your board-binding combination, whether riding it, sitting in a lift chair with it hanging from your feet, or carrying it around. In other words, weight is a very personal consideration. If you are a small person, it's likely that some equipment that feels normal to many will feel rather heavy to you. If that bothers you, shop around until you find something that seems more comfortable to you. It's unlikely that the modest weight differences among bindings will have a meaningful impact on your actual riding performance.
Binding flex is also a personal preference, although there are obvious patterns to people's choices: freeriding = stiffer; all-mountain = medium flex; mostly park = flexy. But as long as your binding isn't soft as a noodle, it should be acceptable (not necessarily ideal) for all-mountain riding, assuming you aren't actually charging like Jeremy Jones.