One of the characteristics of outsider groups/philosophies is that they view/interpret common cultural images and symbols differently than the norm. Furthermore, if you were to give pretty much any intelligent, insightful human a set of anything and ask him or her to come up with a 'story' about how those things relate, they can probably do that (some people are better at this than others).
Give a group of Cathars or people interested in their ideas a set of images and it's probable that someone among them can devise a great story that link those images in a way that relates to their interests.
Did an actual Cathar or Cathar sympathizer actually do this before the 18th century? There is not one shred of evidence to even suggest that anyone did. Could one of them have done so? Sure (we can't know what goes on in people's heads if they don't tell us). And, as we see from all the cultural tarots that modern people have devised (Celtic, Japanese, Hawaiian, Native American, Buddhist, Pagan, etc., etc.,), lots of modern people have done so in light of their favorite worldviews, too, and told us about it.
Could someone have modified the design of a tarot deck to reflect Cathar ideas more clearly in their own mind? What makes this more likely than any other of myriad possible cultural scenarios of the period in question? I don't see a single piece of evidence that makes it more probable than a hundred (or even a thousand) other stories I could come up with.