That would depend on her mission, which is likely to be AntiSubmarine Warfare. If she's intended to operate under an umbrella of shore-based F-15s, then there would be no need for Harriers, which aren't interceptors anyway. Clearly Japan has a need to protect her seaborne commerce, without whch they would starve in short order. But using the far more effective F-15s for CAP would allow her to operate more helos.