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Pokemon XY also has the same thing, where player Serena uses her standard smiling face throughout the entire game in the overworld. None of the camera shots ever focus on her face, though, so it's less noticeable. Rival Serena's a different entity, her expessions already have the grey eye color and fair skin tints applied, so they can't be used with player Serena, as her textures have white, mannequin-like skin and silver eyes, tints are applied when determining skin and eye color to give her those.
May and Brendan in ORAS are able to express themselves more, since they use the same model both as a rival and as a player, and the rival has a lot of expressions to choose from. This also allows camera angles to focus more on their faces.
It seems like Pokemon SM tried to follow ORAS's footsteps in that regard, but it falls flat since You/Mizuki don't have any facial expressions in the overworld.
