it's hard to give an answer to "what do i do about nair" because pretty much every character's nair is safe or punishable depending on their ability to space it.
Vs spacies, you should WD back and dsmash or shield then punish. You *can* get away, it's just very difficult if they're already on you. WD back twice is really good and will almost always give you an excess of space. Sometimes you gotta get counterintuitive with it and WD towards the pressure to escape.
Vs marth, you generally want to dash attack him whenever he jumps. the problem is that with the rest of his aerials, you can dash attack once you see him jump, but nair will stuff that dash attack. so you gotta note that he's doing it and dash attack on anticipation. If you're really getting stuffed, chuck some turnips and try again later.
Vs falcon, there's almost nothing you can do against a wellspaced fadeaway nair. I had a match against Hax a while ago where I got COMPLETELY locked down by fadeaway nair. You NEED to trade dash attack with it. Still, sometimes it just. . . won't work. Remain patient and wait for him to screw up. It's not a proactive solution but it is a way out of the problem.
I've always been for writing very long forum posts, but I doubt people are dying for my matchup advice
Maybe one day when I'm better.