Hmm, Johnny Carson retired from the Tonight Show in 1992.
Taylor Swift was born in December, 1989 and started singing at age 10, several years after Johnny retired. She performed for the first time in 2006 so the dates don't match up that she ever was on with Johnny Carson.
A Google search shows her first appearance on the Tonight Show with Jimmy Fallon was in 2017.
She was probably on with Jay Leno.
A lot of us oldsters still called it the Johnny Carson show for a LONG time after Johnny turned the reigns over to Jay.
Old habits die hard (especially for us old people)
