Quite frankly there’s no way for an AO to know the significance of a karate national competition. And unless you did have a frame of reference ie won first place where the competitors were USA team members, how would anyone know how good the competition was? Maybe it is an open competition where anyone is allowed to compete.
Yes, if you attained black belt while in HS you can certainly put that down as an award on your ECs. If you won some local competitions that might be good for some filler for your ECs. If karate had some sort of ELO rating system like they do in chess or ping pong you might be able to reference that too, but once again there needs to be a frame of reference. In running, for example, one might not win a competition but if they ran a 2.5 hour marathon that would be a frame of reference that would be really good.