Not much impact impact on NX handheld, I don't think. People already have a phone, NX is a barrier to entry.
I'm thinking of this from another angle; Pokemon Go is a gateway for non-gamers, non pokemon fans etc to get into pokemon/gaming with minimal effort. It's hard to imagine there won't be a positive impact on NX sales as a result should there be an extended version of Pokemon Go on NX or some compatibility between Pokemon NX and Pokemon Go etc.
A lot of variables involved there though, and it's far too early to make any real educated guesses here imo.