The aesthetics of musical mashups lie in a particular kind of technical virtuosity and set of listening skills, rather than in the creation of something entirely new or original. The art is to succeed in finding two tracks that fit together musically, resulting in successful songs in their own right.
Mashups are characterized by two underlying principles: contextual incongruity of recognizable samples and musical congruity between the mashed tracks.
Contextual incongruity often creates a humorous effect, which explains why many listeners react with smiles and laughter when hearing a new mashup. In successful mashups, the combination of musical congruity and contextual incongruity results in the paradoxical response: “these two songs should definitely not work together…but they do!”
This according to “Contextual incongruity and musical congruity: The aesthetics and humour of mash-ups” by Ragnhild Brøvig-Hanssen and Paul Harkins (Popular music XXXI/1 [January 2012] pp. 87–104).