I have no idea! I honestly haven’t come across the term but it sounds rather silly.
I remember approximately 2 years ago a friend was very enthusiastically describing to me her confirmation dress. I asked what color it was & she proudly said ‘Mother Mary Blue!’. I found it rather outrageous & silly at the same time. I notice that the viel Mother Mary wears on her head is usually pale blue so I guess she was referring to that! :P