There is reason to believe that Erika was originally a Ghost type Gym Leader during development. Her original sprite and Sugimori sketch depict her with her yukata worn right over left, a style traditionally used only for dressing the dead. With this in mind her closed eyes and blank expression take on a new meaning, indicating that she may be a spirit. Her levitating PokéBall also fits this theme.

Furthermore her sprite was altered in Yellow version to the standard left over right yukata. The only other sprites changed in Yellow were Brock and Misty to match their appearances in the anime, but Erika's new sprite doesn't match her anime appearance at all and was seemingly only done to change her yukata presentation from dead to alive.

The going theory is that she was changed to a Grass type Gym Leader when they realized there weren't anywhere near enough Ghosts to fill a Gym, whereas the Grass type was well represented.

It's also possible that the right over left yukata was a mistake, but this is unlikely given the cultural significance of it in a game made by Japanese people who would no doubt be aware of said culture.