Torkoal did beat Shiftry in the Hoenn League...it was Tyson's too.
And he almost won against Registeel. It is very hard to beat a legendary pokemon and to be so close to winning, it's still great.
And although winning does reflect on strength, that's not all strength of a pokemon can be based on. Although corny, "it doesn't matter if you win or lose, it's how you play the game". How you battle against the opponent can show how strong a pokemon can be. In the case of Torkoal, it battled amazingly. If I never watched pokemon and Overjoyed is the first episode I watched, I would say that Torkoal is super strong just to hold its ground against a powerful legendary. Winning isn't everything you know. May's Squirtle won a lot of times but that's because it battled as a DEM, it was cheap and people hated it for it. Oh, and there is Paul...even though Chimchar won against Turtwig, Paul still called it pathetic (or at least he yelled at it).
So strength of a pokemon doesn't just count on wins, but how you battled, who you battled, and potential. That can show how strong a pokemon is. For Torkoal, it battled greatly agaisnt Registeel, it battled a legendary, and it does have potential to be strong as seen in its battle. And from hardly beating Morrison's Steelix to almost beating Brandon's Registeel...I think Torkoal definitely improved and definitely became stronger.
I could say more, but I'll stop right here...Torkoal is or at least can be one of Ash's go-to strong pokemon like Heracross, Tauros, Snorlax, the rest of the AG team, and the Kanto trio.