To get the Pokemon TCG Pocket Mew card, as mentioned above, you need to complete the Kanto Pokedex, which means getting at least one of every of the original 150 Pokemon in the game. Once you do so, an option will come up in your "missions" tab to complete the quest and reward you with the Mew card itself, like any other mission reward.

There are over 200 cards available to collect, battle, and build a deck with in Pokémon TCG Pocket. These cards range from gen 1 to gen 8, with a small handful of Pokémon from each generation of Pokémon,

Pokemon TCG Pocket's secret missions are the most difficult in the game, and players deserve better for their efforts than what they're given.