>>36950482Mewtwo is before Mew because
1. Mewtwo is a Legendary Pokemon while Mew is a Mythical Pokemon
2. if Mew came before Mewtwo, there would be an empty slot before Mewtwo keeping players constantly looking for the 151st Pokemon, if they aren't aware of Mew's existence they can completely fill the Pokedex with zero blank spots left and 'complete' it.
3. blank spots thing from before except Mew was initially a secret Pokemon that was only mentioned in myths and stories and wasn't confirmed to actually be in the game until distributions began, so having it in the 151st spot preserves the surprise