When you have a bad torque converter the bad torque converter shouldn't cause any damage to the transmission.
However if the torque converter is failing then it can't give enough power to the transmission to keep the gears shifting properly or to propel the vehicle forward.
So you may feel like your transmission is going bad when actually it's your torque converter but sometimes your transmission may be the problem after all.
Check the transmission fluid to make sure it's full and not smelling burnt or contaminated as that can cause damage to the transmission if the transmission fluid is burnt or contaminated and if the transmission fluid is low it can cause transmissions to not shift properly or eventually fail because the bands in the transmission cannot get lubrication properly.
A bad torque converter can cause things such as transmission overheating, strange noises, higher stall speeds, and slipping between the gears.