Over 15,000 have complained to TV watchdog Ofcom after Diversity took to the stage in the first BGT semi final with a dance routine inspired by the Black Lives Matter movement.
Appearing on ITV's Good Morning Britain, Amanda backed the routine and said the complaints proved it was necessary.
"It is extraordinary. I think the number of negative complaints means this conversation is right and fundamental," she said.
Amanda continued: "What hasn't been really highlighted is we have been inundated with overwhelming positive responses, hundreds of thousands of them.
"As the BGT family, and Diversity are very much a part of that, Ashley is doing a phenomenal job as a judge, and we stand by him and stand by that performance 100 percent."
She went on to praise Ashley as an "extraordinarily intelligent man" for his response to the complaints.
Amanda said: "He sends out positive vibes to the haters and the lovers of the piece that they did, and it just shows what an amazing human being he is.
"I think he is saddened by the negativity, but it backs up the fact we need to be talking about it, and we all need to support it and talk about the positivity."
TV watchdog Ofcom has yet to say if it will be investigating the complaints further.
For now, Britain's Got Talent continues Saturday nights on ITV with Amanda, Ashley, Ashley Banjo and David Walliams on the panel.