Daisy makes this statement in chapter 1 of The Great Gatsby, and it's clearly a facetious comment made to mock her husband Tom.
Tom and Daisy Buchanan, Daisy's cousin Nick, and a friend Jordan are sitting around a table at a rather awkward dinner. Several glasses of wine later, the conversation is still rather sparse and awkward, and then this:
"Civilization's going to pieces," broke out Tom violently.
Tom goes on to extol the virtues of a new book he's read, The Rise of the Coloured Empires by "this man Goddard." This book apparently warns white people that black people are getting poised to take over the world, a theory which might have been more credible at that time; today it would certainly be seen as ridiculous and outrageous. Daisy apparently thinks so, too. The theory espoused, says Tom, is that
if we don't look out the white race will be--be utterly submerged. It's all scientific stuff; it's been proved.
The very next line is Daisy's comment to which you refer. We read that she made her comment "with an expression of unthoughtful sadness," then goes on to kind of make fun of him for reading "deep books with long words in them." While her words may not sound particularly mocking or sarcastic, Tom interrupts her to defend his position on this book. The text says he "insists" and he glances at her "Impatiently." This defensiveness is his reaction to Daisy not taking him seriously. Her tone, then, is mocking and sarcastic.