It just so happened that Ukrainians are jokingly called Ukrainians, but the question is: "Why?" It turns out that this has gone from ancient times. Zaporozhye Cossacks in the old days shaved their heads and left their forelocks.
They did it probably according to an ancient tradition. In Kievan Rus such a forelock meant belonging to an elite military class and a noble family. In particular, a Byzantine description of the appearance of Prince Svyatoslav Igorevich has been preserved, indicating that he wore a sealed man "His head was completely naked, but a tuft of hair hung down on one side of it."
For the first time this definition was recorded in the dictionary "Trilingual Lexicon" by Polikarpov in 1704. The meaning of "Ukrainian, Little Russian" for the term "crest" is also recorded in Dahl's "Explanatory Dictionary".
So the word "Ukrainians" originally meant the people wearing forelocks. Over time, traditions have changed, but the nickname for the Ukrainians has remained, acquiring a playful and disgraceful coloration.