I only remember Sophie Tucker as a legendary figure that people might have mentioned when I was a child. I really had no idea of what she did or why she was “the last of the red-hot mamas.” A quick web browse tells me that she seems to have broken down a variety of barriers – a Jewish immigrant woman singing comic songs, jazz, blues, and various other types of songs. She also seems to have been an early advocate of recognition of fat women with songs like "I Don't Want to Be Thin," and "Nobody Loves a Fat Girl, But Oh How a Fat Girl Can Love." And a union organizer in an organization that preceded Actor’s Equity.
No clear answer about the red-hot mamas.