Discussion Forum—A Way with Words, a fun radio show and podcast about language

Here is a quote from Word Detective:

The funny thing about "crackerjack" as an adjective meaning "splendid" or "first-rate" is that although it is most often heard today in the U.K. or Commonwealth countries, "crackerjack" was an American invention. In fact, "crackerjack" was originally a noun, appearing in the U.S. around 1895, meaning "a person of excellence, superior knowledge and ability." The root of "crackerjack" is a sense of "cracker" current in the early 1860s meaning "a remarkable individual" or "an outstanding example of something." This sense of "cracker" was based, in turn, on a very old (around 1460, in fact) sense of "to crack" meaning "to boast or brag." The "jack" element of "crackerjack" doesn't really mean anything -- its role in the word is to rhyme with "crack." Once again, although "crackerjack" meaning "excellent" is no longer very common in the U.S., it was obviously a popular adjective when "Crackerjack" snack food appeared in the early 1900s.

Here is a link about U.S. uniforms.  Even though this one says that the candy got it's name from the uniform, most sources say the opposite, that the name of the uniform came because of the similarity to the picture on the candy.