Jo-Jo is the tiniest member of the Who clan—an already rather tiny population. When the Mayor of Who-ville is desperately running around, trying to get all the citizens of Who-ville to make some noise to avoid certain doom, Jo-Jo is the last person he finds. He's just a little guy, but it's his voice that puts them over the edge and makes them known.
This tot probably represents the individual who speaks up, even when it seems kind of useless. Every hear someone say "every vote counts"? This is Seuss's version of that. When Jo-Jo speaks out, change happens. How very inspiring, little J.