Thats what I do in my games, house rules to make it more believable and dramatic. Level and evolution dont have anything to do with each other. If your Pokémon hates you, it might never evolve. If you fall off a cliff, your level 20 charmeleon just might evolve to save you. Dice and stats are important, but fun is WAY more important!

Edit: the hp natures exist because of how stating Pokémon work. In this game, your base stats determine what order your Pokémons stats must be for the rest of its life. If base hp is 10 and base attack is 9 with an attack boosting nature, your attack can never be higher than your hp, unless your nature also lets you lower your base hp. Also, they added new ones rather than just altered the neutral ones so that 1.) Neutral is still a possibility and 2) it makes rolling natures for wilds super easy with 2d6.