Clothes with dotted patterns became popular during the polka craze of the late 19th century.
In the middle of the 19th century, polka music and dancing was wildly popular in Europe and America. Polka clubs were started emerged and became very popular.
Members were very proud of their clubs and, somewhere, someone came up with the idea of adopting a uniform pattern on clothing worn by women who were members of these polka clubs. Women chose a closely fitted jacket that was widely worn and made of evenly spaced dots placed on a field of fabric of a single color to signify their membership in a polka club.Why Are They Called “Polka Dots”? Why is it Called a “Bikini”? – History Spaces
Bonus Biscuit: Polka-Dot Man
There was a villain in the Batman and Robin comics called Polka-Dot Man. The polka-dots on his multi-colored costume would each transform into various weapons or devices in the commission of various crimes and dastardly deeds.