Hell, I haven't even played through the Teal Mask at all yet. Was it really as underwhelming as people say?
In a way yes, but it depends on what you expect from a Pokémon DLC.
I really loved the DLC characters (Kieran, Carmine and Perrin), I prefer them to the central characters of the base game, with the exception, perhaps, of Arwen. However, the DLC's story, which is short, is quite simplistic and generic, and just an excuse to work on the introduced characters. In my opinion, the story is who is at the service of the sibling duo, and not the other way around.
On the other hand, despite the kitakami map being reasonably well constructed, with some interesting landmarks, and big, the game's content does not make good use of it. The story, despite trying to sell this proposal, does not actually invite us to explore the entire map nor are we well rewarded for doing so.
There are few genuinely new Pokémon and, personally, most of them, although linked to the plot, didn't interest me in the slightest. I don't deny that Ogerpon's terastalization, visually, is cool, but that's it.
Another issue is that the game does not add any new post-game activities, like the Max Lairs from The Crown Tundra. After finishing the DLC events, there is nothing left to do, other than the boring Ogre Oustin mini-game in which you can get a Shiny Munchlax.
Finally, I cannot help but highlight how shameful the kitakami festival is. An Indie studio would do a better characterization and presentation of the event.