To be honest, it was just an outfit for Rowlet admiring Decidueye. I don't think Ash meant to use it as part of the battle. But I did think it was messed up that Hau's own grandfather screwed him out of a win.
well there was a tactic that involved the Decidueye mask... Where Rowlet used the mask to escape the Spirit Shackle (or was it Sinister Arrow Raid??? Hmmmmm...)
THAT makes SO much more sense, but I have a "Counter" up my sleeve... The mask was noticed... By the ref, and NOT taken away for the remainder of the match... Remember, Nanu, the ref, gave it back!
That Everstone also just happened to be seen by Rowlet, and he INSTANTLY went for it... Even AFTER Ash and Rotom Dex tried (and failed) to force him to spit it out. And that action of spitting it out allowed Rowlet to use Seed Bomb... So, it was required... In a sense...