The Barstool Sports personality -- better known by her nickname Brianna Chickenfry -- hit up Madison Square Garden in New York City for fight night Saturday ... when the venue started playing Bryan's "Something in the Orange."

LaPaglia's clearly not happy with the song choice ... pointing her thumb straight down and appearing to boo while her ex's voice fills up the Midtown Manhattan arena.

Also, just listen to the audio ... the person filming and several others in the crowd seem none too happy about the song coming on.

Of course, in the aftermath of their breakup, Brianna's blasted Zach repeatedly ... claiming he was emotionally abusive to her during their entire relationship -- adding that, despite their breakup she's still terrified of him.

Brianna's also alleged Zach offered her $12 million to sign an NDA and clam up about their romance -- though she's made it clear that will never happen.

For his part, Bryan has not responded to any of Brianna's claims ... or Dave Portnoy's who says Zach once kicked the Barstool founder's dog out of his dressing room before a gig.