I dunno. When I battle her on the ship, I feel I'm fighting an entirely different being. I feel she is a complete stranger.
Maybe I'm overthinking it, but just the rival theme playing on the background while our battle is happening is just so wrong. I just feel she will be more menacing.
Also Madame X has her own theme now. Well, kinda. It fits to her though because Cyrus theme, but more heavier.