What imo makes a good prestiger is a good rate of "damage done until it faints" and CP.
As defense and stamina are square rooted in CP-calculation and go directly into damage until it faints, good defense and good stamina are the first point to check. Porigon 2 is good here, but not overwhelming.
Second aspect of attacking power that does not affect cp is moveset. Three points here:
(2) Super Effective
(3) High damage output.
Your Porygon lacks stab, and this missing 25% makes its prestiging performance underwhelming in comparison to real grass prestigers like venussaur or tangela or even SoB-parasect.