Just saying: if being viable in OU is enough reason to actually reside in the tier, explain how Gastrodon (who is ranked just as high as Mamo on the VR) is freaking PU.
And not just Mamo. B rank has quite a lot of mons from UU, BL and other tiers that aren't OU. Looking at Reuniclus, Alomomola, Suicune, Gliscor (this thing even is A- ffs) and Kingdra (although that can be debated with rain slowly dying).
And if Joey just wanted to highlight Mamo's niche, and not deliberately move it up, explain how the series is literally called "To The Top" and he calls out his fans to start using it to get it to top usage.
And again, Mamo's departure is pretty harmful to UU. If he took something like Quagsire (relatively irrelevant in lower tiers) or Gliscor (bound to move up anyway) it wouldn't be much of a deal.