Alola doesn't have high number of powerful trainers but the opponents Ash faced/beat is obviously more powerful,
Ash got a rival Tapu Koko who is beyond champion opponent in games or anime and Ash battle him throughout SM, beside Alan charizard x not a single rival Pokemon even during their league can hold a candle against Tapu Koko, Gladion from the start show really powerful trainer even had a legendary, Kukui another rival is literally like in game Champion, Ash beat Misty's mega Gyarados with almost ZERO damage on Pikachu, he beat UBs Nihilego in it's home world in less than half series which is the feat Ash only achieve in last season in past, battle Dia and his Mythical Zeraora like 3 times, Ash beat Gladion to win the league and Gladion is more powerful than some league winners like Tyson and Virgil while Kukui+Tapu Koko was literally the high champion opponent.
Compare Ash gym performance to Kahuna, Ash shown fodder trainer in DP-XY first gym battle where he lose badly for some bs reasons while Ash beat both of Hala's trials like a real professional trainer and Hala was even more powerful than his game counterpart during his trials, already said how big difference Ash shown in Misty's gym battle, Nanu's battle was use to shown Lycanroc strength and endurance which is shown even later series, Lycanroc also recover some health with the berry Tapu bulu given, Olivia grand trial shown Ash's amazing skills in battle and even use his past series strategy to win, Ash shown his creativity from DP series with Electroweb.
Those feats are factually higher than past series.