All that can presently (December 2022) be gleaned about Mabel Thomas from sheet music covers and by Internet searching is that she was presumably an American actress/singer who has been featured on just one work namely Sleep My Kinky Headed Coon (1900) words & music by John J. Fitzpatrick, and published by Raymond Music Pub. Co, Providence, Rhode Island, USA
Her portrait on the left is taken from that same work.
Surfers are welcome to contribute any further information about this performer.