Keep in mind she died in 1994, so chances are somewhere around 100% that she once ate a taco at some time in her life, especially since she'd been to Mexico. It might be a more interesting question to ask if JFK ever ate one. But yeah, for Jackie, we can assume she most ceratinly had a taco at least once in her life.

That said, as for having one in America earlier in her life, keep in mind that tacos as we know them are a relatively new food, and at least in the US they have always been a working-class food. As you think of the ubiquity of tacos today, they're still pretty rare in more upscale places, and usually as a single fish taco on a restaurant, or the nicer taquerias serving local craft beer in newly gentrified neighbourhoods. 1951 saw the opening of the first Taco Bell by Glen Bell, who tends to get credit for making "Mexican" (Mexicish?) food mainstream in America. Restaurants followed, and while these were primarily in the Southwest during the time of JFK's life, they were popular in those regions, regions where JKO spent a lot of time over the years.

So even taking out trips to Mexico, or the food she ate in Mexico not being tacos, or her living to 1994 when tacos were much more common around the world than they were in the 1960s, the chances that she'd tried one are still incredibly high.

There's a book Planet Taco: A Global History of Mexican Food that's a fun and informative read, if you're interested in, well, the history of Mexican food. There's a lot more to the history of the taco (and American Mexican food) than what's required for the sake of your particular question, but you seem like the kind of person who might like to read up on that. It's interesting stuff, and ranks right up there with the history of a lot of well known Asian foods in America.

See also:

• Would Frank Sinatra have gone out for sushi?