Mel B isn't winning any hearts and minds in the "America's Got Talent" audience ... sources tell TMZ, she just got booed AGAIN ... this time over an arguably racist joke.

Sources at the NYC auditions tell TMZ, a comedian took the stage moments ago and began to tell a joke about how black taxi drivers are more entertaining than other taxi drivers ... and Mel buzzed him.

She said, "I found it really offensive. There was no need to do a stereotypical black joke. It was not funny." But the audience disagreed ... booing Mel for hitting the red button. 

Fellow judge Howie Mandel then asked the audience if they thought the joke was offensive ... and the audience replied with a loud NO.

Howard Stern said Mel lacked a sense of humor and Heidi Klum chimed in, saying, "There will always be jokes made about being black and white ... I agree with you that we should laugh about it."

So we gotta ask ...