Symbolism, Imagery, Allegory
Jo-Jo may be a little guy, but his voice is the one that eventually saves the Whos:
The lad cleared his throat and he shouted out, "YOPP!"
Jo-Jo's shout represents that even the littlest and smallest thing can make a difference, even if you don't believe that it will. Every vote counts, right?
Take McCarthy-era politics as an example of just how dangerous it is to keep quiet. During the Cold War, anyone who seemed different or suspicious was put through the wringer. Should the American people have just let the government crackdown on regular ol' folks? We're going to go with no.
Jo-Jo's voice reminds us that if something is terribly, terribly wrong, it's your responsibility to use your voice. In fact, in the grand scheme of things, it could be the very thing that matters the most.