Given that this sprite is more or less directly based on the original sprite, there isn't a huge amount to say about this sprite. It works, and provides a new subtype of enemy that people can probably find a use for.
The main issue with this sprite is that, because it is based directly on the original SMW sprite, it lacks features that would make it otherwise consistent with things like the SMB2 sprites. That being said, that's just a consistency issue and it falls within the context of SMW fine. If you're looking for a rideable version of the Ninji you will want to look elsewhere.
If you ever want to update this sprite to add more features, maybe give it the ability to stun Mario when it lands on the ground. Maybe even let it kill/stun other sprites like a yellow shell Yoshi.