What do dill and scout learn from mr. dolphus raymond, the white man who has mixed children and a black mistress in tkam?

They learn that Mr. Raymond is only pretending to be drunk.

His bottle contains cola, not alcohol, as everyone assumes. He believes if people think he's a drunk it will be easier for them to understand his behavior.

In reality, he is not drunk. Instead he believes white people are hypocrites and therefore prefers to live in the black community.