Sweet merciful Batman, you people argue about the most POINTLESS crap! I mean, who even GIVES a **** whether Yamask using Shadow Ball makes the episode filler?! Me? I'm waiting on what Seger said before dismissing this as a filler, just because this could potentially mean it's not.
EDIT: Bulbapedia at the very least seems to consider Yamask knowing Shadow Ball a major event. Not saying it is, but again, waiting on Seger's words.
Please stop complaining about Pikachu losing to "weak" Pokemon at the start of each new region. Its power is drained at the start of each new region, and anyway, it's equally bad storytelling for Pikachu to plow through all of its opponents.