Wieliczka is generally quite safe for solo female travelers. As with any travel situation, it's important to take general safety precautions like staying in well-populated areas and being aware of your surroundings, but many travelers find Wieliczka to be inviting and secure. It's a historic city with a well-preserved salt mine which attracts many tourists, and as such is quite accustomed to welcoming visitors. You should feel quite confident and safe in traveling to Wieliczka.