I have to be honest, I don't understand the point of a non-constructed playable mythic planeswalker. I'm particularly confused by the fact they had to squeeze her in - she's been so central to this storyline, I can't see who apart from Liliana has greater priority in the set.
I mean, I get bad cards need to exist, and I get bad fares need to exist. But a planeswalker where they know full well she isn't constructed playable is just meh? I guess it's for kitchen table magic and flavor.
* Jace, Nahiri, Sorin, and Liliana were considered important story characters from the start, I assume.
* Including a red/green werewolf planeswalker was considered high priority, so Arlinn was included.
* Tamiyo was probably meant to play a very minor role in the story, but it got expanded as they wrote it out. They had probably already developed Kaladesh+ planeswalkers taking the above five into consideration, so if they wanted Tamiyo to get a card, it couldn't be something that would drastically affect the Kaladesh planeswalkers.
As a result, Tamiyo was developed to be interesting but not particularly strong, a card that might be used a lot in Friday Night Magic but not in tournaments. So, this card is meant for casual play.