Some UFC fighters hate Monster and other sponsors because they often have to jump through various hoops to represent the brand during fight nights. They are required to hold up a Monster can or wear a t-shirt to get their sponsorship bonus. Many MMA fighters surely want to focus on getting their job done and earning their pay. Requiring more tasks to get paid a fair wage only adds more difficulty and frustration to their job. You'll also see that fighters have Monster water bottles to drink before the fight and between rounds. Monster doesn't actually make water in their normal commercial products, but their brand is seen on drinks is a major part of their advertising effort.