I have heard of the "good enough mother". This is from attachment theorist Donald Winnicott. I think it means that in order for a healthy relationship to develop between the mother and child, there has to be some degree of attunement, but it doesn't have to be perfect. If there is some attunement some of the time, it can be "good enough" and the child can grow up with a secure attachment. By extrapolation to therapists, it has been said that they need not be perfect to help the client, just good enough.

I am not sure if what your T meant by "good mother" is the same as "good enough mother" though. Maybe she can provide a good reference for you to learn more.