Some say that during amavasai tharpanam, if the mother is still alive, it is enough to perform tharpanam for great grandmother and grandmother on the mother side. Is this appropriate? Please clarify.

If the mother is alive, tharpanam should be performed for the father, paternal grand grandmother, and paternal great grandmother. Cannot be performed for one’s mother. Must be performed starting with father’s mother.
Notes
Hence these groups of women would not fit with the pitru varga, one need not worry.