Side-B could be her phasing through objects being that kitty does suffer herself when phasing. Similar to Fox's. Up-B could be levitation because she can do that with her power. B could be a pitfall touch since kitty can phase others she touches. This ability works of platforms and ground but in air could put people in footstool fall.
All just suggestions for B moves. The down one doesn't seem bad as it is. You could leave her B and make her Side-B pitfall. You're the creator after all.
Also Kitty is especially weak against energy attacks when phasing and she does use phase guns she made herself. She can also damage things she phases through. Just a bit of back story.