Will Map Editor be added to Stadia?

I would like to know as well, I am waiting to buy the game until I know which platform will be more mod friendly

Steam is probably the more mod friendly, and Stadia the worst as it is streamed game it also depends of Stadia:

• Will there be mods on Stadia?
  While it’s an interesting idea, mods support on Stadia is not something we can achieve on our own, so we cannot make any promises.

Stadia is lifecycle is doomed to end soon, just like Necrophages crushed and gangbanged by any other race.

You must be really idealistic and have a lot of money to throw out if you want to invest into Stadia.