Yes, but the smartphone app would have to be active and would require people to receive it from others with apps, thus creating two issues
1) Two sections of player, 3DS carriers and phone carriers - Segmentation
2) The issue of density of 3DS players remains the same
1) How does it segment when (afaik), Streetpass games are single player?
We already have two classes of players - those lucky enough to live in an area that is population dense and has people who cary their 3DS on with the wifi on and people who live in areas that are less dense. For those in the second group, the Streetpass games are mostly pointless
2) The issue isn't as much the density of 3DS, there are a lot of 3DS all over. The issue is people who carry their 3DS around all day with wifi on. It is incredibly easy to install an app that just passively collects Miis. I really don't see how this is a huge burden on a user.
ETA: I don't think this is a case of either/or. It's really between people who just carry smartphones vs people who cary smartphones+3DS.