Columbus was never laughed at because he thought Earth was round. As Gerhard Prause points out in his book from year 1986, frequently repeated legend about Christopher Columbus being mocked by the dignitaries of the Royal Court of Castille is a widespread misconception, that originated in the first half of 20.th century.
The idea that educated people at the Columbus’s time believed in flat Earth, still remains one of the most prevalent errors in Western teaching. In reality, idea of spherical Earth was widespread since the third century B.C. Historian Jeffrey Burton Russell points out, that ”with only a few exceptions, no educated person in the Western history who ever lived after the Empedocles, believed in the flat Earth”.
At the time of Christopher Columbus, shape of Earth was no longer discussed, however the exact size still remained a speculation. Columbus’s estimate of the Earth’s circumference was around 17,000 miles(28,000 kilometers), In contrast, figures estimated by the dignitaries of the Royal Court of Castille, based on the calculations of famous Florentian mathematician Paolo Toscanelli, were very close to actual earth’s circumference, that being 25,000 miles(40,000 kilometers).
And this, not the shape of the Earth, was the reason why Christopher Columbus was ridiculed.