>>44615442The mono ear is intended to resemble hair tied back like a ponytail or topknot. Scorbunny is sporty, big floppy ears would go against the core design philosophy of hyperactive, sports obsessed, gonna-be-the-best attitude. Raboot goes floppy ears to emphasize the "determined, but cool about it", invoke training uniforms, and cinderace quickly changes that as the pokemon comes into its own and gains confidence.
For the record, I'm not a fan of the line's progression seeming more like "kid grows up" instead of "companion monster gets stronger." It is what it is, though.