I'll bring up the obvious since no one else has.
Charmander participated in, what? 3 battles before it evolved? And spent all of about four episodes as a Charmeleon? Nobody complained about Charmander's rapid growth spurt. Hell, I was excited to see Ash with a Charizard because Charizard was my favorite Kanto Pokemon at the time. Popplio is barely any different.
But of course, Charmander gets a free pass because Kanto and Early Installment Weirdness, right? And Popplio is evolving quickly in a series that isn't trying to be battle-heavy to begin with. So I don't see what the problem is.
I'll agree that yes, maybe Popplio should have evolved last year amongst all the filler. But hey, Popplio is so bland that when it evolves is irrelevant, right?
It would still spend most of it's time in the background no matter what form it's in simply because it has the misfortune of belonging to Lana and not Ash or Lillie.
TL;DR: When Popplio evolves or how long it stays in whatever form really doesn't matter. SM isn't a battle-heavy series anyway, so battle experience is largely irrelevant.