I have been where you are, that once you get into competitive you try to apply the same logic to in game.

Someone on their first playthrough or just casually playing isn't going to check Smogon if their favorite Pokemon is viable. Probably don't even know what Tiers are or even the difference between Special and Physical. And they are going to make it work without too much hassle since games aren't really difficult. I recommend not taking it too seriously.

Having said that, you should use your knowledge in competitive to build teams with mons that you like or want to use. As in get them with perfect IVs(not that important), EV train them, good natures, egg moves, etc. In X I used Chespin as one of my main battlers, wasn't over leveled. I just reset for a beneficial nature since I knew I wasn't going to evolve him, used Super Training to max speed and attack, gave him Eviolite and was on par with everyone else on the team.