The phrase motherhood and apple pie denotes a core principle, value, belief, characteristic, aspect, etc., of the U.S.A. or its citizens.
—Cf. also (as) American as apple pie.

The earliest occurrence of the phrase motherhood and apple pie that I have found is from the column Making ’em Dance, by Jim Dance, published in The Miami Herald (Miami, Florida) of Sunday 15^th April 1956:

It isn’t a habit I wish to foster, but perhaps I should explain myself on the subject of baseball. This because, all over, minor paroxysms of joy are erupting in the sports pages, gaining strength as the season approaches. I am a dissenter, and for this minority opinion I have been adjudged variously of everything from imbecility to Communist leanings.
I didn’t mean no harm, originally . . . it’s just that I stopped to think about the game. This hasn’t happened to too many, and numerous are they who go around agreeing with everybody else that it’s “The National Pastime.”
In this class are businessmen who recall their lost youth and the last game they saw, 40 years ago; 17,000,000 bowling participants who forget they outnumber the 16,000,000 repeaters who just watch baseball; all city officials, who turn up opening day and never quite manage to see any of the other 153 games; and a catch-all group of unestimated millions who have been led to believe disagreement is the same as attacking motherhood and apple pie.

More generally, the nouns motherhood and apple pie have been juxtaposed in enumerations of things and persons exemplifying U.S. values. These are some of the early occurrences that I have found:

1-: From the column Assignment America, by Inez Robb, published in several U.S. newspapers on Thursday 6^th November 1952 for example in The Franklin Evening Star (Franklin, Indiana)—the following is about Balinese dancers who were touring the USA:

You want to know what the extraordinary Balinese girl-ballerinas, ranging in age from twelve to fourteen, think of the famous Rockettes ¹, an American institution revered with motherhood, apple pie and Joe Di Maggio ²? The Rockettes who even bowled over Paris on a post-war tour?
“They are very well disciplined, but what they do can scarcely be called dancing,” is the solemn judgment of Ni Gusti Raka, the 12-year-old star of the troop, and her sister-dancers, after seeing the Rockettes kick up their heels at Radio City music hall.

¹ Founded in 1925, the Rockettes are an American precision dance company which has performed at Radio City Music Hall, in New York City, since 1932.
² Joseph Paul DiMaggio (1914-1999) was a U.S. baseball player.

2-: From the portrait of the U.S. actor Edward Mier Mayehoff (1909-1992), who was playing Jarrin’ Jack Jackson in the CBS-Television series That’s My Boy—portrait published in The Pasadena Independent (Pasadena, California) of Sunday 1^st August 1954:

[Eddie] has a very definite philosophy about his work inspired by his feeling and sympathy for the people of whom he feels he’s one. And he looks just like the stolid Kiwanian or Rotarian he is in real life and hails from Norwalk, Conn.
“These little guys are not Babbitts and buffoons,” says Eddie. “They’re the back-bone of America. Without their committees and Chambers of Commerce nothing would get done. You’ll never find me kidding their—or I should say—OUR morals or motives, only kidding manners. It’s one thing to kid a man’s or family’s conventions and another to kid his or their convictions.
Similarly, Eddie hates Communism, loves his country, church, family, motherhood, apple pie and meetings.

3-: From the column Labor looks at, published in Labor’s Daily (Oklahoma City, Oklahoma) of Thursday 19^th April 1956:

For a period, four elements of American life were as unassailable as the Vestal Virgins—publicly, at least.
They were motherhood, apple pie, Grace Kelly ³ and President Eisenhower ⁴.

³ Grace Patricia Kelly (1928-1982) was a U.S. film actress.
⁴ The U.S. general and Republican statesman Dwight David Eisenhower (1890-1969) was the 34^th President of the U.S.A. from 1953 to 1961.

4-: From the column Over the Rim, by Carolyn Gimpel, published in The Oklahoma Daily: A Student Newspaper Serving the University of Oklahoma (Norman, Oklahoma) of Friday 4^th May 1956:

It is time to expose a myth, undermine a fallacy and reassure the average of righteousness. All of our lives we have been told—“do not put off until tomorrow what you can do today.” […]
[…]
That favorite feminine fictional lead, Scarlett O’Hara ⁵, had a lovely way of looking at life. “I won’t think about it now, I’ll think about it tomorrow.” This relieves you of the necessity of facing unpleasant situations. Nothing is really quite as unpleasant tomorrow as you think it is today.
Of course, there is one prerequisite for doing this successfully. It is better to face hordes of unpleasant things than to worry about them. Worry is a pointless mental exercise which results in ulcers, hypertension and basket weaving. And as anyone can tell you, there is not much of a market for baskets.
In answer to this viscious [sic] half-truth which is one of our most treasured adages, all we have to do is quote a few others equally popular and equally meaningless. For example, “an ounce of prevention is worth a pound of cure.” It is obvious that this could mean nothing other than what it says.
If you take this folksy warning to heart you will realize that you should never do today what can be done tomorrow because that’s certainly prevention and there you are with a pound of cure.
Along with apple pie, motherhood and the flag, these heartwarming sayings will make life easier for you. Clearly, as long as we have these intellectual tidbits supporting us, it will not be necessary to think. And thought, we have been assured, is dangerous, unless it is the right thought.
Therefore, turn to page 606 in your Handy Little Sayings Manual and the choice for today could be none other than that wonderful phrase and staunch bulwark against communism and decadence in general—“The wages of sin can be a good day’s pay.”

⁵ Scarlett O’Hara is a fictional character in Gone with the Wind (1936), by the U.S. novelist Margaret Mitchell (1900-1949), and in the 1939 film of the same name.

The following photograph and caption are from the Meriden Record (Meriden, Connecticut) of Saturday 21^st June 1958:

A FOURTH OF JULY PICNIC or roadside snack without a hot-dog these days is considered by some people as being unpatriotic as denying Mom’s apple pie, American motherhood and baseball. Leslie Caron ⁶, petite star of MGM’s cordon bleu musical romance, “Gigi,” celebrates the Fourth in a safe and sane manner.

⁶ Leslie Caron (born 1931) is a French-U.S. dancer and actress.