The gibberish the Mayor speaks before slamming his arm into the wall, is the same gibberish Steve Carell used when he played Evan Baxter in Bruce Almighty (2003) and Evan Almighty (2007).
The Horton quote "I meant what I said, and I said what I meant: An elephant's faithful one hundred percent," is actually from "Horton Hatches the Egg", the first Dr. Seuss story featuring the enduringly faithful elephant.
The first portrait that the Mayor shows to JoJo is, in fact, a Who-style caricature of Theodore Geisel/Dr. Seuss.
After the snow falls in Whoville, the children can be seen building a "Grinch" snowman.
JoJo is also the Mayor's son in Seussical the Musical, which also combines both of Horton's stories.