I'm not actually sure whether she said it, but it wouldn't surprise. There are some pretty good recent historical analyses of Antoinette, and she was vilified in great measure because she was a foreigner sitting on the French throne. Sympathetic figure, actually.
Yeah, as i said below, when thinking about cultural memes and politics, the thing is to pick one that is instantly recognizable to your audience and that carries the connotations your interested in -- in this case, overbearing privilege, arrogance, and indeed the implicit acknowledgement that there could be violence in offing if the peasants get really ticked off and begin to take things personally.
This is why so many devices work well as the basis for these poasters -- Kirstin Dunst, because her expression is priceless, Foghorn Leghorn, because he's known for sounding off, Captain America, because, he's. . .well, he's Captain America. You get it.
This is a true statement. But, you know as well as I that in politics what counts is not historical accuracy, but rather the current cultural relevance of the meme, and as you say, currently, the "let them eat cake" meme is very relevant.