Arlo is intentionally simplistic both to make him stick out from a complex environment and to make him immediately physically relatable. (source) Honestly, I hated it at first, but it's growing on me.
Oh hey! I'm glad this topic exists! Fan art question: Has anybody seen any fan art that's done in the same simple style as the animation? I keep running across really cool Gem OCs, but they're always detail cluttered and thus can't fit into my headcanon (if that makes any sense).