I've battled Every ingame trainer in ss and I havent seen 1 dunsparce. is it NU?
"NU" is a pokemon tier by Smogon for competitive battling. It means Dunsparce is never used in real battles (because it's weak).
It has nothing to do with the in-game trainers and battles. Usually you see nearly every pokemon because all trainers have different types for different reasons.
Yes. Dunsparce is NU according to smogon.
But remember that the tiers are just made by smogon, not the pokemon company, so it has no influence on the in game trainers.