To write such a fanfic without being offensive, it's crucial to respect the Pokemon universe's values. The 'slutty' aspect should be more about a style or a character trait rather than something demeaning. You can explore how this style affects their relationships with other trainers, Pokemon, and the community. Also, don't objectify the character. Instead, show their agency and how they make choices based on their own personality and goals in the Pokemon world. For instance, a trainer might dress in a revealing way to stand out in Pokemon shows, but also be a very kind and skilled trainer.