Why would you hesitate to call it art? What is your definition of art?
There’s a difference between cartoons, illustration, graphic art and fine art. “Art” is fine art. It’s possible for cartoons to come to be regarded as fine art, see Roy Lichtenstein. It’s also possible for illustration and graphic art to come to be regarded as fine art, and perhaps this is happening with Norman Rockwell.
However, overt cuteness and sentimentality tend to put me off, with him just as much as Hallmark greeting cards or that guy who painted all those Snow White & The Seven Dwarves-looking cottages beside creeks with glitter in the paint, what was his name? Thomas Kincade?
Norman Rockwell did produce some fine art, perhaps inadvertently but intent does not matter, ultimately, His popular work was cartoons, illustration and graphic art, rather than fine art, however.