Does likening a person to a certain nationality, such as Sudanese due to skin color, for the purpose of joking without insult, fall under backbiting (gheebah), mockery, or ridicule?

There is no harm in describing a person by a certain nationality as long as there is no intention of belittling or mocking them. This is evidenced by the Prophet's (peace and blessings be upon him) description of Moses as being "as if he were one of the Zutt men," who are a race from Sudan or a type of Indian.