Is improvisational jazz to impressionism art as smooth jazz is to realism art?

Not sure I am following your analogy. Are you saying smooth jazz is more like photo realism in painting? And that improvisational jazz is more like impressionism? I’m not sure I’m capable of answering that.
I was not listening to jazz for years, despite having shelves of straight-ahead stuff, some of it considered "important," but what got me going again several years ago was stuff in the ’70s done by some of the players who had serious credentials but no market. So, what I began to explore was more eclectic, personal, less commercial stuff that eventually became very collectible because it was only issued in small runs by small or private labels. Strata East and Nimbus West come to mind-- some great material on those labels.
Cecil McBee became one of my favorites- a very melodic bass player who appears on a lot of so-called spiritual/soul jazz and some stuff that is classified as "free jazz." I don’t like complete cacophony, but appreciate it more as an accent or element of contrast -- Pharoah Sanders is known for this multi-phonic squawk from his horn but he can slide back into a melodic line which hits the center of the sweet spot in a way that is sublime.
There’s a lot out there to explore. I think one of the virtues of modern jazz (or any type of music for that matter) is that you can educate yourself by listening and the process is pretty enjoyable. And there’s a fair amount of information out there to tap into.
For me, it isn’t an intellectual pursuit, or one that I consider an investment. But, the more edgy stuff is something I have "ears" for now, partly due to exposure and digging down into a vein that has proved to be rich, e.g. McBee’s work. I guess I like straddling the line between what would be considered post-bop and "free" jazz, although I’m always happy to listen to Art Pepper (Cecil appears on one of his late records "Today"), or Bud Powell or other greats from the earlier eras.

I totally get what you are saying. For instance, I like some Miles, but then I get lost in some of his free-form. I can appreciate what he is doing, but after one or two of his more improvisional stuff, I start to have trouble following it. My opinion is, you have to really sit and listen "hard" and not lose track of what he (or others) are saying with their improvisions - whether in art or music. In relationship to art, I believe it’s similar, for some art, you have to "study" and think hard about it to get what the artist is trying to say.
I don’t feel dumb for not sometimes understanding and I don’t put myself down for it. Some art I get (Banksy, Van Gough) others not so much (Lichtenstein, Duchamp). I get most Miles, and some other improvisional jazz guys, but some I don’t. Actually there is classical I feel the same way about, and rock. A good example of rock, I can listen to 1-2 songs of Yes, but then I find it gets "annoying". Even though I know Yes makes amazing music. Same with Jethro Tull and Frank Zappa - great music, but only a little at a time and not all of it, but that’s me, not the quality of the artist. However, as my nickname alludes too, I get all of the Dead. And I know many who can’t stand what they do (my wife sadly). I cannot explain why. But I do continue to re-try those musicians and artists every once in a while. I think someone else here called it "active listening". Some music and art are simply not for casual listening no matter what. I’d suggest to you to try it in small doses and also play one song 2-3 times in a row. Sometimes repeated listening/exposure helps to dig into it deeper.
I am not saying I am right or wrong. about this or any artist, just telling you how I hear and see it myself and understand where you are coming from.

Oh, as an add, what I listen to in Jazz that's smooth but not mush, try Stanley Clark, Return to Forever, The Modern Jazz Quartet.

To a certain extent you are on the mark when it comes to music that has obvious scales, tunes & meter being roughly equivalent to representational art.  And again, it is indeed a matter of taste whether or not you prefer your music to be straightforward. The thing is, the world is extraordinarily complex. A lot of it simply does not make sense at first or even second blush. I gotta say, though, that once you begin stretching your aesthetic sensibility and allow yourself to become comfortable with things that might not be crystal clear at first glance, you'll uncover a whole universe of treasures. 

Nice post, @deadhead1000- I don't think anyone seriously thinks they can convince a person to like a certain type of music, and the added difficulty with jazz (not quite as much as a factor with rock during its heyday) is that jazz performers appear on lots of records as non-featured artists. To a lesser degree true with rock, but some of my favorite work is actually that of the side persons/session players on a given recording. 
I grew up around people who would follow the Dead; my listening of them told me they were consummate, and could jam blues/folk/rock/ endlessly at a high level of technical and musical proficiency, but it really is a long trip, isn't it? (Some of these jazz tracks are entire sides, not making that a measure of anything). 
On Tull, having come up on them from the beginning, "Stand Up" is so much the template for what that band did in creating a men in tights medieval balladeer meets crushing rock (the guitar wielded by Martin Barre was pretty gnarly); This Was is more blues and jazz and doesn't reflect the general direction of the band (though Aqualung, commercially and musically, may be the album of broadest appeal). But, at the end of the day, just using that band as an example you can find different periods associated with different overall sounds/styles from the band. Sometimes, your point of entry affects your perception, too. 
In connection with jazz, I really didn't pay any attention to any of these records at the time they were released. They were only discovered by me after many excursions into more unfamiliar terrain, sometimes, with knowledge of one or another musicians who appeared with them for that particular recorded performance. 
Maybe my mantra should be listen more, talk less. But, then you'd never see me. :)

@edcyn- +1.
Bill Hart