Actually, performing well at NASS can provide a SMALL bump in admissions. It's not large as great candidates can't attend NASS for various reasons (schedule, cost, other SS, etc.).
For this reason, not attending is not a negative. Let me repeat that, not attending NASS does NOT hurt you in the admissions process.
If you do attend NASS and do poorly -- this can be a result of poor attitude, poor fitness, etc., it CAN hurt you. You are being evaluated. That said, USNA recognizes that the people doing the evaluating were plebes about a week earlier.
Thus, it is rare for a bad evaluation without justification to carry much weight. IOW, unless you're a total screwup and everyone noticed, a mediocre NASS performance won't hurt you. Probably.
My advice:
(1) Be in shape when you go. Yes, you can retake the CFA. But if you can't keep up with the activities or are always the last to accomplish anything due to being out of shape, that will be noticed. A great CFA later on can make up for it, but you start out with one strike against you.
(2) Don't be a whiner. No one likes whiners. If you don't like a certain element, keep your mouth shut and do your best. It won't last forever.
(3) Dont be a suck up. The only thing worse than whiners are brown-nosers (suck ups).
(4) Do your best at all times. It's easy to tell when people are really trying. You may not be great at everything. Welcome to plebe year! If you give 100% in all you do, you'll be cut a lot of slack.
(5) If you happen to be great at everything, help out your classmates who aren't as strong. Be a team player and team leader rather than a show-off. No one likes show-offs either.
(6) Ask questions of others and yourself. NASS is merely a taste of plebe summer. If you really hate NASS, you probably won't like USNA. If you love NASS, you have a decent shot of liking USNA.