Chick tracts were (and still are) quite popular with evangelicals in the US, mostly of the fire-and-brimstone variety and of Baptist or Pentecostal persuasion. So in that sense, yes, they were successful to appealing to their audience. Churches have been getting attention for distributing them as recently as 2014.

Outside of that, though, yeah, they've been fringe since day one. The Chick Publications website talks quite a bit about the resistance that Chick tracts faced from bookstores and publishers, even those that were Christian. Jack Chick was very disapproving of Christian denominations aside from his own flavor of evangelical hellfire sermonizing; he was rapidly anti-Catholic in particular and this shows up in his comics very often.

Catholics and certain Protestants like Anglicans/Episcopalians, Methodists, Presbyterians, etc., were probably meant to be part of the intended audience because Chick wanted them to convert but I can't imagine Chick tracts convinced many people to become born again considering the absolute vitriol thrown at non-evangelicals within the comics. So Chick tracts ended up only really being popular with people who already believed what they were saying.

This was kind of hard to answer from a historian's viewpoint because I honestly don't know how much has actually been written about Chick tracts within a historical context, but they're an interesting artifact of American evangelical culture, if you want to call them that.