Hmm, interesting question..because the classical idiom was kinda the standard popular idiom, then. Franz Schubert today may be a classical composer, but he was very much a popular musician- he'd play for gatherings, back up singers, was famous for improvising on piano.

One guy that might fit your definition is Carl Michael Bellman, lived 1740-1795 in Sweden, played a metal-strung lute thing called an arch cittern, composed and sang lots of beautiful songs and was a great entertainer and improviser on his instrument. In that, he resembled a pop star of today. (He also had a kind of tragic, hard life and drank too much: which seems to often go along with being a pop star today). But still, you have to admit that what we call classical music is present in his melodies.

Now, some musicians did take advantage of folk melodies. Edward Bunting collected and released lots of Irish music, including some that had been played on the very-Irish wire-strung harp...and Thomas Moore set a lot of his poems to Irish melodies. But if you look at their published works, those melodies are arranged in that classical style...Bunting's Irish tunes were set for piano, with the left hand part sounding like it could be from Haydn . Moore's songs were published with full piano accompaniment sounding ( to our ears) very similar. So, TL:DR, because classical was the pop music of its day, it was hard for somebody to be a pop star outside of it.