Of American-English origin, the phrase (as) American as apple pie means typically American in character—apple pie being here a symbol of American motherhood and traditional family values.
—Cf. also the Australian-English variant as Australian as meat pie.

The earliest instance of (as) American as apple pie that I have found is from Seeing Circus From Behind the Scenes Reveals Why It Makes Kids of Us All, by Edwin C. Hill, published in The Sun and the New York Herald (New York, N.Y.) of Sunday 4^th April 1920:

Twenty years or so ago, when the Kaiser was cranking up the German military machine for the madman’s scheme of making Bagdad a suburb of Berlin and otherwise preparing himself for an unhappy old age in Holland, he sent a Von Somebody to this country to travel with the Barnum & Bailey circus. The idea was to study the smooth, swift system by means of which an enormous aggregation of men, beasts and material was handled and transported with perfect economy of time, effort and money. There has never been anything comparable in fact or fancy to circus efficiency, except the methods employed by the djinn Slaves of the Lamp that picked up and set down whole cities at the behest of Aladdin.
The Prussian Excellency returned to Wilhelm and the Great General Staff carrying bulky note books full of efficiency recipes, all borrowed with German precision from that distinctly Yankee institution the circus; and the All Highest, who knew a good thing when he saw it, directed that a good many ideas evolved from experience and pure gumption by circus experts of America be incorporated in the system of handling the masses of men and material of the German army.
It is rather interesting to know that something of the perfection of the German mobilization and of the swift smash through Belgium and northern France was due to the use the Germans made of ideas as American as apple pie. Possibly they wouldn’t have had to back up at the Marne if they could have hired the Ringlings to handle their French tour, because, although ideas are great things, the men back of them are a lot more important.

The phrase gave rise to the term apple-pie American (the words apple and pie must be hyphenated as a compound adjective modifying American, itself used as an adjective or as a noun).

The earliest instance that I have found is from the Democrat and Chronicle (Rochester, N.Y.) of Saturday 13^th September 1952, which published First Thing Family Wanted: Sailor Back After 2 Years Celebrates with Home Cooking, about a sailor who, on his return to Washington, went over to his mother-in-law’s with his wife and two sons “for a big turkey dinner with lots of ice cream and fresh strawberries”:

Suppose you’d been out of the United States for 2½ years. What’s the first thing you’d do when you got back?
The chances are that if you were a good apple pie American, you’d head for some home cooking right away. And that’s just what Anthony Ciresi and his family did when they returned a few days ago from Tony’s Navy assignment with the American Embassy in Rome, Italy.

The adjective apple-pie has come to qualify other nouns; for example, in The Pittsburgh Press (Pittsburgh, Pennsylvania) of Monday 30^th March 1970, Richard Starnes wrote the following of the Texas billionaire Henry Ross Perot (born 1930):

He is a sincere believer in old-fashioned home, mother and apple pie patriotism.