The big names (Pacha, Hi Ibiza) lean smart-casual and door staff can be picky. Beach bars and the more underground clubs (DC10, 528 Ibiza) are far more relaxed, so trainers and a T-shirt are fine.
If you’re booking a VIP table, note that most clubs require men to wear long trousers in the VIP areas specifically: shorts that were fine on the main floor won’t get you past that section. Outside the VIP tables, shorts and smart casual are accepted pretty much everywhere.