This phrase became so famous that Krugman occasionally references it.
And I reference it in my play DARK MARKET:
Olivia
A friend of mine knew a producer at the DC public television station. I have a Ph.D in Applied Mathematics but I felt I hadn’t applied it to help anybody much. I liked the idea of making math fun for kids. You know what Barbie said: “math is hard.”
Larry
Barbie? You mean like the doll?
Olivia
Yes. They made a talking Barbie and one of things she says is “math is hard.” Not exactly encouraging little girls to get into math.
Larry
Do you honestly think that’s the problem? Not enough encouragement?
Olivia
That’s a big part, sure. What do you think it is?
Larry
When my daughter was little, I bought her some trucks, because you know, I wanted to be politically correct. And she told me that one of the trucks was the mommy truck and one of the trucks was the baby.
I also reference Larry Summers' infamous math and science are hard for ladybrains speech in the above passage. He really did use his daughter's play habits as an example of how girls are different from boys, which means we shouldn't be surprised if they don't do STEM careers as well as boys.
But after doing some Googling it turns out that Barbie didn't say "math is hard" she said "math class is tough" and this was controversial back in 1992.
A talking Barbie doll criticized by a national women's group for saying "math class is tough" will no longer utter the offending lament, Mattel Inc. has decided.
The Teen Talk Barbie, which costs about $25, will remain in stores. But the computer chip that randomly selects four phrases for each doll will now pick from 269 selections, not 270.
The American Association of University Women attacked the math comment in a report on how schools shortchange girls. The doll hit store shelves in July.
They may have discontinued the phrase in 1992 but it lives on in social memory. Albeit not accurately. This piece from 2006 discusses the historical morphing of the phrase.