To the general audience, not in the slightest now and not in the slightest then. With the rise of broadcast technologies, Czechs wanted to listen to foreign songs, but any sort of culture, pop or otherwise, originating in the capitalist world was strictly banned in the Soviet world. At the same time, the Iron Curtain not only crippled people from escaping their own country, but also made international copyright laws something of a very vague suggestion, so "western" songs were basically a free idea bucket. And thus, an extremely prolific cover song culture was born.

Various singers young and old, regime-convenient or underground, covered everything from traditional folk songs to Goombay Dance Band to Elvis Presley to Elton John. There's even a wikipedia article on the subject: [link]

Some of the songs were rather diligently translated with meanings mostly intact, like Eldorado (W. Matuška), some get somewhat shifted like Mánie (Blíženci band) with text dancing madly between trying to rhyme with the melody, making sense and being faithful to the original (and usually failing all three of those), and some are complete and utter nonsense that only cares about finding the best rhyme for the original title, regardless of whether it makes sense or not – my absolute favorite example being Boney M's Rasputin, which becomes Žena s glóbusem, with lyrics narrating the story of a bossy woman who sits at home "spinning the globe with poise and decorum" and commands her man to dance to her every whim.

However, there was a particular kind of subculture that the John Brown song (and singers like Matuška) is a part of, and that is the culture of tramping (again with a helpful wiki article that sums it up better than I would.) While John Brown himself would not register even as a blip on a cultural radar for general public (even the tramping one), there was a very eager audience for country and bluegrass songs that could be sung at a campfire with a guitar or two (in a similar vein, look up the band called Greenhorns, they had quite a lot of such songs.)

Matuška himself almost certainly knew who John Brown was, however, as he had access to articles of the US culture, history and popculture, and a lot of his work is of the fairly accurate translation kind (he later moved to the States when the pro-Soviet regime began to consider him too much of a nuisance). Audience was not as lucky, because even if the name sparked curiosity in a few, access to information on the subject was extremely scarce.