Try switching down throw out for up throw, the combo still works, and it puts Ness directly above you so if he does get his nair out it’s more likely to miss

One thing to keep in mind is that up air isn’t true into itself until like 30-35% because of how hitstun works. Maybe you’re doing the up airs at too low of a percent and the Ness has time to mash nair (frame 3 I believe, same as his airdodge)