First, this quote tells about the different lives the boys have lived. Jem stands out in a crowd; he has grown up with love and support, in a community where everyone knows and respects his family. Jem views the world from a standpoint of superiority. Dill, on the other hand, feels belittled by the world around him. He has struggled against a poor home environment where he is neglected, and the situation of being small for his age and gender. He feels "laughed at".
Therefore, Jem looks at clowns and thinks it is ridiculous for anyone to act out so much in order to get attention. Dill looks at clowns and feels that it is a good opportunity to feel superior to his audience.
Yes, the argument could be made that Jem is pessimistic. He takes a person whose goal is make others laugh, and insists that the person is sad and hopeless. Dill, on the other hand, sees the situation as an opportunity to excel and stand out, a much more positive view.