At the end of the day, the arc wasn't really dumbed down by Lillie or Sophocles' presence, they didn't hog screentime by doing nothing, it was just not their moment in the limelight. There are far better examples in the series where an episode didn't work having so many characters (eg. villain episodes where only Ash fights while everyone just stupidly stands there concerned, though those always happened even in the three companion series).
Sophocles got his screentime before and after with some limelight concerning his new Charjabug, while Lillie's main arc was obviously upcoming about ten episodes later. The choice of characters made sense given Akala was their home turf in the games and neither Lana or Kiawe had gotten limelight in a fair while at that point. The only one who really got gipped was Mallow who was pretty much hand waved dead weight in the Grassium trial episode, while Lana and Kiawe got to be the stars in their respective trials.
If anything SM at least has character balance and omits some characters from many scenarios, the only problem I have is that they could maybe have more cases it's beneficial to one of the supporting students. I like Ash but his character is already being used fully this series, and some of the others feel rather lacking. Lana seldom gets an episode she's the driving force, even the episodes she's the 'star' have now started to revolve more around a Pokemon or threat with her just overlooking, and I've just given up on Mallow altogether. I've also noticed that overdoing Ash's screentime also requires him to sometimes be more flawed than usual, to the point of taking on compressed vices at times, when it would arguably be more beneficial to develop foibles for the other characters (the moral for the recent Rotom episode arguably would have fitted any of them better than Ash). It's all about moderation.