Being a powerhouse doesn't state that you need to have an extremely powerful trainer,it is only required that your Pokemon has proven to have excellent offensive presence. Yes in-game these Pokemon aren't exactly powerful but in the anime they have proven to wield an above average amount of power seeing as the anime=/=in-game
His Palpitoed was of great usage in the battle against Clay as well as some other battles
His Unfezant was shown to be very skillful in battle as it was able to defeat Skyla's Pokemon even after it had been weakened before it was sent out
His Krookodile is with no doubt Ash's main powerhouse in Unova standing very close to his famous Charizard that was able to defeat a legendary Articuno even after it's wing was frozen. His Krookodile is one of the few Pokemon that actually managed to defeat Iris's powerful Dragonite
Though his Snivy hasn't even evolved yet, it is very powerful being able to hold itself against 2 rather powerful fire types(Emboar and Heatmor) in a double battle
Many people always state that Ash sucks at battling and even though it seems to be the case 70% of the time, he has proven to be a rather powerful opponent later in the series. Yes he is weak when he starts his journey in a new region but unlike "us" he doesn't get a whole preview on every single Pokemon in the region. Another thing that states that we can't compare anime to real life and in-game