shyt, that is a subject worthy of its own thread.
Trump was a promoter for Affliction MMA. They only had two cards. Vitor Belfort(the Brazilian you mentioned talking that MAGA nonsense) scored KOs in both events. His involvement in MMA has garnered him support amongst some MMA fighters the same way it has in boxing(ie Mayweather and Tyson are both Trump supporters)
A MAJOR reason for this nonsense has to be the Gracie family. It's hard to overstate their influence in MMA. Here's a hypothetical that should help point it out. Take a very important aspect of boxing...let's say footwork. Imagine if through effective marketing and whatnot, the vast majority of the boxing world was convinced that the ONLY way to learn proper footwork is to train with one particular family(or someone they've personally trained). That's basically what the Gracies have done in MMA.
Here's an article detailing some of the family's
history, including founder Helio Gracie's ties to a fascist groups in Brazil. It's a long read, but it also goes a long way towards answering your question.
He Trains Cops. His Jiu-Jitsu Family Has Deep Ties to the Far Right.
That article is just scratching the surface. It doesn't even cover the Gracies' classist fueds with Oswaldo Fadda or luta livre...nor does it cover Rener's juelzing about Eric Garner's murder.