Yes, in my Summer of Italian... Food Porn, even the most American of holidays has an Italian food connection. And of course it's pasta.
Returning to England, these young men were enamored with all things Italian: the rip-roaring history; the high fashion with tall, decorated wigs; the classical architecture; and the delicious food... like macaroni. To be fair, anything would taste delicious to young Britons raised on Marmite and haggis. But I digress. Other older, less worldly Britons liked England being English and derided these internationally-minded young men as foppish dandies before the label "foppish dandy", existed so they took to calling them "Macaronis". Sort of like how we call today's instagram-ing, bacon-loving population under 35, "Hipsters".
In the 1700's, the British in Britain looked down on American colonists as being uneducated, rough-around-the-edges and home-spun. I mean, who did these colonists think they were trying to fight a war against the mighty British army? Therefore, during the American Revolution, British troops made up a song to make fun of this uncouth American.