Torterra is one of the biggest disappointments in the anime.
Pokemon that were strong while Turtwig and Grotle were suddenly weakened.
Speed is more important than strength in anima. Slow pokemons could not make a big impact.
Ash has always done well with fast pokemons.
Slow pokemons like Torterra, Torkoal, Boldore are not for Ash.
Snorlax is perhaps the only slow pokemon Ash can use.
Ash's torterra keeps getting bashed but don't you realize that all of Torterra's opponents were incredibly strong and powerful?
It's first loss was against Bertha, a member of the Elite four. I mean, other than AG, none of Ash's Pokemon has even displayed power remotely close to Elite four tier. Even Ash's Sinnoh Ace, i.e Infernape was easily destroyed by Flint's infernape.
Torterra's second loss was against Volkner who was written as this extremely powerful gym leadewr who lost interest in battling simple because of how strong he was and he also rivalled and trained with an elite four member in the past. So from a narrative standpoint, it would make sense to give Volkner the first win in the battle to give Ash the underdog status. I mean, Volkner didn't even use his Ace and Infernape had to activate Blaze to win. So Torterra losing against Volkner isn't that big a deal.
It's last loss was against Paul's Drapion. Paul was a trainer just as competent and possibly even stronger than Ash. His whole strategy was to use his first 2 pokemon to reveal Ash's team and then try to sweep with his Drapion. From a Narrative standpoint, I guess this battle is probably Torterra's poorest performance and something it could've won, but it's other 2 losses are against an Elite four member and a Super Powerful gym leader.
So I honestly find comments about Ash's torterra being weakened post evolution simply absurd.