Smash popularity =/= overall popularity. Greninja was barely talked about before it was revealed and had a very tiny support thread. But it is undeniably the most popular Gen 6 Pokemon And I wasn't even talking about its popularity.A standout. really?
Go to /vp/ for example. Find 1 Tapu Koko thread. There isn't any.
No one talks about Tapu Koko. Is there even a Tapu Koko support thread?
And whether or not a Pokemon is popular now, the game plan was made in 2015. Pokemon Sun and Moon came out late 2016. Depending on the time, Rowlet probably wasn't even publicly revealed.
In the game, the story both begins and ends with Tapu Koko, you meet it before you meet your own starter Pokemon. And it is seen, heard or mentioned many times until the of the game where it personally challenges you.
In the Anime, last I saw wanting to beat Tapu Koko in battle is one of Ash's driving forces to getting stronger right now, it has notably taken very special interest in Ash and was the one who gave him both his Z-ring and Electrium Z. He even taught him a few Z-moves.
He was designed to be popular. That's what I meant by it being a standout. Despite being one of four island Guardians, Tapu Koko from the very beginning was a standout and a league above his fellow guardians.