>>32402492Basically for going through the story, your Popplio is completely fine.
Once you get to the post game and things get a bit more serious, you might want to breed a new one and go for a Modest nature since it's generally better. I say in the post-game since through certain tricks you will have ways to manipulate easily which nature the Pokémon you are breeding will have (and even the gender through a thing called "swap breeding"). At that point you will have to take into account also IVs and EVs, but I suggest thinking about them later. This is outdated but should give you an idea of what they are:
http://www.smogon.com/ingame/misc/evs_ivs 
