Now, I wonder if there's a point in discussing this today. Notice that the only things we all can definitely agree to be valuable art are old. Almost none were created when I was alive, and most are much older. At the time of their creation, a lot of them were decried as "mockery of art" and such. So perhaps we should leave that subject to our grandchildren. From what I've seen, valuable art generally gets remembered, the rest doesn't. This isn't very subjective, since something getting a place in collective memory "just sort of happens". While this is an incredibly conservative criterion, it might be the only way to definitely answer such a question considering how subjective the subject is. Such an answer would still be pretty subjective, but there's no more objective answer than that.
If you ask me, FS (and FRED, by extension) has a good chance of passing the test of time. It already stood up for an amazingly long for a video game, and TC could help keep the awareness up. It's already quite well known in space sim circles, and when video games become a more widely respected medium, it has a good chance of being included into the canon of digital classics, complete with original copies selling for huge sums of money.
Otherwise, we can conclude that FREDing is a form of art, because it fits a dry and rather broad dictionary definition. But as I mentioned, this answer is rather trivial and doesn't really tell you everything.