I was around on the internet when this was happening, so I can tell you the real story. Some people went around posting those signs, printed at home on A4 paper, around college campuses during a time when social justice issues were at a fever pitch in popular culture. Some very sensitive-to-social-justice-issues and sensitive-to-potential-indicators-of-people-being-insensitive-to-social-issues people saw them and became upset on Twitter. I think the signs usually got taken down, but that's what usually happens to signs.
Some people took this as a sign that an evil fifth column was going around trying to win an invisible cultural battle by putting up signs that had no racist content but that implied that someone around town would be willing to put up a racist sign if they had had the chance and the chutzpah.
Other people took this as a sign that the evil first through fourth columns were in such active pursuit of dissenters that they would get mad on Twitter and take down meticulously inkjetted pieces of A4 paper if they so much as implied that dissent was an option.
Few reasonable people took this as a sign that it was, literally, not "OK" to be a white person (what does that even mean? if it's not okay how are they supposed to stop?), but because taking down a sign can be taken to mean that the janitor disagrees with what those cultural mercenaries at Kinko's printed on it, that take got passed around a lot.
I believe the popularity of the sign lies in its ambiguity. You see, the mainstream discourse never went as far as to say "it is not OK to be white." It was hinted at, in a more or less subtle way, through the "white privilege" (some corporations actually have obligatory trainings on this) and in thousands of indirect ways making white people guilty of the skin they were born in.
So for some people the message of that poster was infuriating. You can't openly disagree with the literal meaning, but on the other hand, many people do it internally. They believe that white people as the descendants of slave owners should inherit the collective guilt of their great forefathers (even if it was a family of a third wave of immigrants from Europe that never owned any slaves). The very fact we are even discussing that is telling in itself.
It is okay to be who you are. It doesn't matter your race or nationality, your sex or gender or creed. It's okay to be white. It's okay to be black. It's okay to be Asian. It's okay to be First Nations, Native, Indigenous. It's okay to be male or female or intersexed or trans. It's okay to be Catholic or Protestant or Sunni or Shia or Jewish or Orthodox or Baha'i or Zoroastrian or Buddhist or Hindu or Taoist or Shinto or any other creed.
For all people everywhere, it is emphatically okay to exist.
If you disagree with that, then I think you need to re-examine who is engaging in supremacist politics.
Some people took this as a sign that an evil fifth column was going around trying to win an invisible cultural battle by putting up signs that had no racist content but that implied that someone around town would be willing to put up a racist sign if they had had the chance and the chutzpah.
Other people took this as a sign that the evil first through fourth columns were in such active pursuit of dissenters that they would get mad on Twitter and take down meticulously inkjetted pieces of A4 paper if they so much as implied that dissent was an option.
Few reasonable people took this as a sign that it was, literally, not "OK" to be a white person (what does that even mean? if it's not okay how are they supposed to stop?), but because taking down a sign can be taken to mean that the janitor disagrees with what those cultural mercenaries at Kinko's printed on it, that take got passed around a lot.