Dill is quoted as saying:
"Well I'm gonna be a new kind of clown. I'm gonna stand in the middle of the ring and laugh at the folks.”
This was a retort to Jem, who explained to him that he has it all wrong in wanting to become a clown, because clowns are, in essence, sad and that it is the people who laugh at them.
Keep in mind that Dill is reacting to the general tension and racial division at Maycomb, not to mention at Scout's scoffing Mr. Gilmer's bad attitude by staying that "he's just a Negro" which shows that Scout may be getting stained with the callousness of Maycomb. Therefore, Dill could only mask his frustration and sadness with laughter, and this is where the clown idea stems from.
He wants to laugh at people insteadof them laughing at him. This symbolizes his growth as a maturing man and how this reflects upon the society among them. He is referencing the Tom Robinson case and how people are being so inconsiderate and dumb regarding this case based on his color of skin instead of the true facts presented in front of them.
Dill career choice to be a new kind of clown is showing a great sign of maturity. Dill says he wants to be a clown that laughs at people instead of people laughing at him. This means that Dill wants to laugh at people (society) because they are being so dumb in the Tom Robinson court case.