I am told that it is a slur for white people, though I've never heard it prior to a few days ago, never outside of Imgflip, and as a white person, it does not offend me at all. Someone was comment timered for a comment which contained the word "mayo" and that's where the whole thing started.
Anywho: "Mayo" has become shorthand for uptight and biased site-level moderation that strains to find superficial TOS violations in liberal comments, while overlooking grave patterns of abuse and harassment from right-leaning Imgflippers.
e.g. I was timered for a comment here on PoliticsTOO which consisted of nothing but the word "Mayo" repeated about 36 times, which I posted in solidarity with the #Mayo movement here.
As for me: I could get into the harassment and abuse that's been directed at me specifically over the past several months, that Imgflip has done little or nothing to correct, but I wouldn't want to bore you. I know other liberal Imgflippers could report similar things.