Mario Yamasaki might be the most hated referee in MMA. That’s a very subjective statement and can’t exactly be quantified, but he has taken a lot of heat for his actions (or inaction) during his 19-year refereeing career.
Most recently he allowed a fight between Priscila Cachoeira and Valentina Shevchenko to go on way too long, then defended his actions afterward by saying he allowed Cachoeira “to be a warrior” and that’s why he didn’t stop it earlier. His actions led to UFC president Dana White basically writing him off, saying that he hoped Yamasaki never reffed again.