honestly they're not needed for the story(haven't beaten the game yet). IVs and EVs are used to help in Competitive battling. If you want you CAN do EVs/IVs in the story, but I wouldn't recommend it. Also I would choose IVs for the storyline since there's no horde battles, (yet) in Reborn.
they don't really matter in story mode as Shushup said, just play and enjoy the game. You could try to get the nature you want, they're useful and not hard to obtain the desired one
EVs, EVs are much more easily controlled than IVs. IVs take days and weeks to get right, EVs take only a few hours. As such, few people will have outstanding IVs, so it's okay if yours are the same. But anyone actually trying will have correct EVs
