Sailors’ middy blouses were a very popular style for boys (and girls) at the turn of the (20th) century and were especially popular in the 1940s. Plenty of old drawings, such as in advertisements and story illustrations, show boys in sailor suits and middies. It was simply a common article of clothing, in style at the time that Donald Duck was created.
1950s patterm
From an antique clothing site: “The ever popular sailor suit for children was a standard throughout the ages starting about 1840. The fashion was both popular for boys and girls. Even adult clothing reflected some of this influence. However, the greatest receiver of the sailor suit fashion was worn by younger children.
A highlight of this fashion was very popular during and after World War II.”