Within reason, the clubs will not refuse people with tickets. The hip and the less hip are allowed to mingle without fear of contamination and whilst attracting attention with your outfit may not be easy, do feel free to try. We are big fans of those who try to embrace the free-spirited nature of dressing on Ibiza from decades past, where an "anything goes" policy was the only rule.
Having said that, the VIP lounges normally don't allow entry with shorts, flip-flops, beach or sportswear, so if you are planning to go VIP, we highly recommend that you dress up! The same is true for the more sophisticated nightclubs like Lío. Ushuaïa and Hï Ibiza also do not allow entry with flip-flops, swimwear and football or basketball T-shirts.