Mamoswine's battle track record isn't great because A: It's trainer was a Coordinator, therefore she didn't get many outside battles and by the time Mamoswine started listening, Dawn was getting shoved to the back by Ash and Paul's time demanding story. B: She's not the main character so the development of her Pokemon are not as good as Ash's, despite what many complain about Ash and Dawn were not equal's in everything. This is why Mamoswine didn't get Charizard treatment. Charizard was a big deal because it was Ash's Pokemon and he is the only one who matters. Dawn was replaceable. But for a Contest Pokemon Mamoswine was alright in it's double battles, at least Dawn found a role to put it in and since Sinnoh contest were trying to break away from raw battles it was impossible for it to have a sheer muscle battle. I don't know why people don't see that.
With that said after the butchering of BW2 Mamoswine doesn't even feel like a powerhouse anymore, just a jobbing punching bag you use to train against. Mamoswine was decent in DP but beyond that he's probably in Torterra's level. I think Piplup is the real powerhouse of that team. Maybe when Quilava evolves it can take Mamoswine's place. After all as a Typhlosion it has a strong chance of getting starter bias.
RIP anime version of Glovehead. Pokemon Adventures will save you.