Was the music of Mozart, et al typically known by (or even relevant to) the average person of the 18th century? Would someone living in Europe but far from Vienna have known about Mozart and/or heard his music during his lifetime, or was it primarily a sort of luxury item for the upper classes?
Obliquely related, were there regionally famous musicians that were considered red-hot in their day (a hypothetical Elvis of Glasgow, let's say), but which have now been forgotten except to historians?
Was Mozart's music known to the average person? Probably not, if you mean the average working man or woman. Serious "classical" music then as now was more a middle- and upper-class sort of thing. Here's /u/caffarelli - who knows far more than me- addressing a similar point relating to opera.
However, among those who did take an interest composers such as Mozart would be well known throughout Europe. Even in the age of stagecoaches composers and performers would tour- Mozart went round most of the European capitals as a child prodigy, Haydn came to London twice in the 1790's, Chopin, Weber, Paganini..(a little later those three of course).. lots of them.
Even those who didn't travel so much -like Beethoven who barely left Vienna- were known abroad. The Philharmonic Society of London commissioned Beethoven's 9th symphony and works by other composers and I'm sure other cities had institutions that did the same.