She is only mentioned in Luke, in two verses. This is all the information there is on her. Here is Smith’s Bible Dictionary entry for Joanna:
The name of a woman, occurring twice in (Luke 8:3; 24:10) but evidently denoting the same person, (A.D. 28-30.) In the first passage she is expressly stated to have been “wife of Chuza, steward of Herod,” that is, Antipas, tetrarch of Galilee.
The wife of Herod’s steward made her a person to be noticed, I’m sure. Perhaps she was mentioned the way a church today might point out the “wife of the governor’s secretary.”