Pokemon is a very easy game, you can beat it with literally any permutation of Pokemon. But you can definitely optimize you team to beat the game better.
Think of it like a speed run. For a speed run, the question isn't "will he beat the game?" That's only a goal by proxy. The actual goal is some other, meta-level achievement: beating it faster than other people.
I can confirm we play different variants of pokemon in that case. I play a variant called Pokemon Showdown where all the pokemon ever created are thrown toghether for you to browse through and create your ultimate team. Because there are so many monsters in that 1 game, the community had to create leagues, which are meta sets that dictate which pokemons can be used in your team comp and which are banned because they are either OP or Underpowered for their specific league and going cross league with pokemons is an easy way to get stomped....like really hard.
Also if we are just talking about single player rpg versions of Pokemon,where unlike showdown you CAN pump health potions in your monsters like crazy.....there is a limit to how much you can steam roll through the game...resistances in that game simply do not allow for that kind of bull....I mean you can beat a water pokemon with a rock pokemon,but only if that water pokemon uses electric attacks and has a speed advantage over the water pokemon,otherwise it will be a very painfull fight for the rock pokemon.
