Also, the Pkm the player used had a high attack stat. It got STAB, was dynamaxed, and was at +2. Despite these facts, Nessa's dynamaxed signature Pkm still survived the fighting move, a type it was naturally weak against!I'd like to point out that by the time he beat Nessa (2nd Gym) Bede had caught up and even surpassed him by a level or two.
Unless her Pkm has some ridiculous stats, it is very likely that it has perfect IVs and, possibly, EV distributions.
This would mean that even if your Pkm is somewhat overleveled, it doesn't mean much when you face Pkm with those IVs and EV distributions since your Pkm usually have modest IVs and an unideal EV distribution.
Also, Nessa's other Pkm have low stats since they're unevolved. Of course, if you use a Pkm with a very high attack stat and a dynamaxed STAB move that boosts your attack by 1, it's going to be easy, especially if it's only the second gym.
I wouldn't be surprised to see stronger and more challenging trainers the more you progress in the game.