Humor has historically been tied to the mores of the day. The Yellow Kid was predicated on what people thought was funny about the immigrant Irish. When you're different in a society, you're funny.

This quote is saying that humor has always been closely connected to the customs and beliefs that were accepted by the people of a certain time period. For example, the Yellow Kid was a comic strip that was funny because it made fun of the Irish immigrants who moved to the United States and had a different culture and customs than the people already living in the country. The comic was funny because the Irish were different, and humor often comes from making fun of differences.