Until the last few years, she was hardly mentioned. Her existence was more implied than stated as they taught that all people are spirit children of Heavenly Father. (If we are his literal children, he needed a wife.) In recent years, she is mentioned more frequently.
Dictionary of Mormonese
Mother in Heaven
Dictionary of "Mormonese"
Stop talking past each other. Gain a better understanding of the words that are unique to Mormonism and the differences of shared terms between Mormonism and Christianity.