It's overall bad in that MU. There is enough downtime from the first hit to the second hit for MK to Nair OOS everytime even when spaced well. Second hit also is shield grabbed easily, and because the hitbox is primarily horizontal it's hard to compete against MK's obvious horizontal spacing superiority. I don't use it that much unless they are at that 50-65ish % range where weak Nair combos into Fsmash every time. Too early, and they Dsmash/hit you after second hit anyways. Too late, and you just get a weak Nair hit no setup afterwards.
I would sporadically use Nair, and would use his other aerials a lot more. The one exception maybe being Nair OOS over other aerials after a MK fair that hits your shield. Other moves that hit your shield you have better punishes like Fsmash, Dair, fart, etc