The movie is an amazing critique of materialism.
The kangaroo is the abrasive, berating skeptic, dying to keep everyone around her in the same slough of skepticism she is in through intimidation tactics and condescension. She dogmatically asserts her worldview when presented with evidence it is false; "If you can't see it or feel it, then it CANNOT exist!" In the end, she is ultimately proven wrong, and is shamed publicly even by her own son whom she, earlier in the movie, exhibits a willingness to give up to exterminate Horton and his opposition to her materialism, despite claiming she does everything for him.
Somehow, I think this was lost on the rest of the audience.
