Is reviling purity considered disbelief (kufr), especially if the mother means the purity of the earth, taking into account that she might be ignorant that purity is an act of worship?

It appears that the mother was ridiculing what the son imagined to be purity, not the Shariah-compliant purity. She intended to object to his excessive scrupulosity and extremism in matters. Therefore, she is not considered a disbeliever. The son is advised to resist and turn away from obsessive thoughts.