Since you mentioned Mozart I'll talk about Vienna a bit.

Part of the reason why you weren't able to find sources on "popular" music of the 18th century is that the line between "popular" and "serious" music was very fuzzy, if it even had been drawn at all. What even is popular music, anyway? There's nothing inherent in the music that makes it "popular." Rather, popularity comes from widespread dissemination and public approval of the music.

There's an book chapter by Sarah J. Adams called "'Mixed' Chamber Music of the Classical Period and the Reception of Genre." In it, she examines reviews of chamber music in a publication called Allgemeine Musikalische Zeitung. In it, she observes that most of the reviews make a distinction between music written for Liebhaber (amateurs) and for Kenner (connoisseurs). This would seem to indicate that some of this chamber music would be considered high art, and would be too complicated, daunting, or advanced for novice musicians.

Within the domain of chamber music in particular, Adams observes that music which diverges from traditional formats (e.g. a string quartet of two violins, a viola, and a cello, in four movements, etc.) has been seen by music historians as "popular" music. She quotes a 1968 critic as saying about Mozart's A Major Flute Quartet:

This is an artless melodious piece, in which each movement quotes, as if by way of parody, a popular contemporary melody.

I'm also reminded of the scene in Don Giovanni where the title character is preparing to host a dinner and listening to the band he has hired. The band plays some popular tunes from other operas: Sarti's I due liganti, Martin y Soler's Una cosa rara, and, in a bit of joke, a selection from Mozart's earlier hit Le Nozze di Figaro. So operatic arias would have been popularly known too. (Amadeus, while not intended as a historically accurate movie, gets this part right: even at the lower-class vaudevilles they are playing snippets from operas.)

It's a bit difficult to compare a time when there was no radio or recordings, when music was ( despite a great Italian influence) very national, and the one of today with a huge mass international mass market, artist recordings...A real hit would be one that everyone wanted to sing themselves, or play variations of at the keyboard, and so there would be sheet music sales.. I would suggest a good English example would be Henry Carey's Sally in Our Alley. It was a very popular song, reprinted many times, became a country dance tune, had parodies made of it...and , perhaps a bit like today, most of the sheet music was pirated, so Carey himself earned very little and died quite poor.

Well, Pop music really came about when music became widely available and sellable to the masses around the early 20th century. Even then, the pop music industry really only came about after WW2, and even then it wasn't as full form as it is today until the early 1980s.

Back in the 18th century, there were a few famous musicians who were fairly widely known, but almost nobody had actually heard their music as there was no way to record the music. There were plenty of people who I'm sure had the notes of the music of these famous people, but they were mostly just rich nobles who could afford piano lessons.

The simplest answer is that there was none. Western popular music was a genre that developed in the 19th century, as the production of music shifted from one of based on patronage to one based on commercial viability. Popular music, in this sense, is music whose approval is measured through consumption. However, there were certainly "popular" genres that developed during this period--Mozart is the culmination of the ascendance of symphonic music (over opera) as the favored form of Western music. Lastly, there is evidence of the opera (or the chamber) as having all different groups together, with the rowdy crowds (the lower classes) enjoying music (quite rambunctiously) in the parterre, the floor area of the theater. So, as Ragleur noted, the distinction between popular and other types of music is not apparent (that would come in the 19th century).

Sources: Derek Scott, Sounds of the Metropolis Richard Taruskin, Music in the 17th and 18th Centuries James Johnson, Listening in Paris