For what it is worth, whoever signed my son's LOA wrote something to the effect of 'The midshipmen tell me you did a great job over the summer...hope to see you in the Brigade next year' by hand under the signature. Maybe he just wrote that for kicks, but I wouldn't go so far to say that what you do in the summer doesn't matter at all. His company commander sought out my wife when we picked him up at the end and told him 'he would really fit in here.' He doesn't have to say that; a simple 'nice to meet you' would have sufficed.
However, what you don't do if you do go to NASS probably matters. That is, don't set a mattress on fire and fling it out a Bancroft window while mugging for Youtube. That might hurt your chances.
As you will all learn as you all go through life, someone is always paying attention to your performance. Don't freak out; but put your best foot forward. If nothing else, NASS is a great opportunity to bang out your CFA.