It is such a time-honoured practice, that in spite of its definite folklore character, there is a Latin term for it: quodlibet, translating "what pleases". Google it with ca. 237000 results.

Like many folk traditions, it has also had an influence on high-brow culture, notably in Renaissance music. Tunes that would not fit together originally, were tweaked by composers. The result was likely to be funny, particularly if songs of different genres were combined, but could also be genuine art.