"Mixed" means _any_ difference between the parents, that the speaker thinks would traditionally be the same.
It is usually "mixed races," but it could also be religion, national origin, or "ethnicity." For example, a marriage of a Christian and a Jew is often called a "mixed marriage." A child of a "native-born" and a "foreign-born" parent can be said to be of "mixed parentage."
A search shows me that an old book describes a person named in the Old Testament, Jeroboam, as being of "mixed parentage" because his parents belonged to two different tribes of Israel: his father was an Ephrathite and his mother was a Mizrite.
To know exactly what it means in your source, it would be helpful to have more context.