>>40618193If strong Pokemon and trainers couldn't lose one odd match occasionally, Gym battles wouldn't make any sense since no novice trainer could ever beat someone who does it professionally everyday. Headcanon dictates that they train weaker Pokemon to fight trainers, but that's not how it works in the anime most of the time.
Now the champion of Alola's ace losing to a kid and his new starter is dumb as fuck, there's no denying, but we can't act as if more experienced trainer always win either.