Toise.
How important would you say popularity is in character selection. We say it a lot, but it's not something that could be measured, so it's not something Sakurai could really pay attention. Despite that, we just seem to "know" who is more popular, so I guess he could just "know" too. Then we have guys like Geno and Waluigi, who are really popular for whatever reason, but they're not going to happen. So popularity by itself isn't the greatest determining factor.
I used to think popularity was just something that could get Sakurai's attention, and if that choice was particularly interesting, then they had a good shot. It's simple, and that's usually best, but I'm not sure it's really the best way to look at things.
So I guess a more open-ended question would be what criteria is actually important. One could probably just say "whatever Sakurai likes," but it's better to ask "why does Sakurai like this." I MEAN WE'RE ALL JUST TRYING TO THINK LIKE SAKURAI HERE.