Reasons to sweeten ground beef macaroni casserole
The only reason to sweeten a dish like this is personal taste, you could add less or leave it out, it's not essential for a beef casserole. It could be a substitute for molasses, although why you'd add 1/2 cup of molasses to a beef casserole it beyond me. 

As for why it's in there who knows? It may be when the recipe was first made that some of the ingredients added bitterness or sourness and the sweetener was added to offset it. Some brands of canned tomatoes can be pretty tart, although 1/2 a cup of corn syrup seems a bit....excessive to counteract that. Or maybe her mom added sugar to get her kids to eat it. Or maybe someone in the family owned stock in Karo. Perhaps she served it to people she didn't like in hopes they'd go into a diabetic coma. You'll probably never know.