If he did MamoswineTTT right away, nobody would be complaining about it. Mons rise all the time due to RMTs or similar being posted and then spammed and the TTT series wouldn't be any different if he didn't start it with an unviable meme mon like Ambipom.
If mons rise because or RMTs that's because those RMTs are actually good. Ambipom rose because one guy THOUGHT it was good, and a skeptical look on the battles he did with it show that it absolutely isn't. Mamoswine is decent in OU and finds its place on quite some teams, but its niche is not big enough to actually reside in it, especially considering the tier has so many good Ground-types already (Exca, Garchomp, Staplefag Lando...)