I'm a web designer. The first rule in design is KISS (Keep It Simple Stupid).
How about writing a novel? Is simplicity fundamental or just an option?
The answer to this depends a lot on what you're setting out to achieve.
In my experience, clarity is most highly valued in genre writing. Plot is often very important in genre writing. Genre writers often try to keep their writing structurally simple, to the point where the reader can forget that they're parsing words and focus on the story.
Literary writing (as ambiguous as that term is) tends to be less obsessed with simplicity or clarity. A significant percentage of "literary" work is at least partly an experiment in language. When you begin using language in new and strange ways, you're purposely making the text harder to read, but you're doing it to achieve a specific effect.
There's nothing strictly wrong with either approach, and there are even works out there that employ both techniques. Hemingway is the classic example of simplicity to the point of sparseness, but he uses that structural simplicity to achieve profound effects. Joyce employed dense, challenging prose, and he is greatly admired for the layered complexity of his work. Some readers enjoy a variety of styles, others love one and hate the other.
So, to answer your final question: yes, simplicity is an option, not a requirement. Your personal style may lead you in one direction or the other. You might go back and forth between projects, and certain plots or characters may cry out for a particular approach.
Don't be afraid to experiment. Writing multiple drafts of the same story, in different styles, can often be a helpful exercise.
I think the core of the novel has to be simple - In general, I believe in the rule that you should be able to explain the basic premise of your story in one sentence. But I think you can build onto that core with as many complications and elaborations as you can come up with, as long as you (and your readers) don't lose sight of the core idea.
From a purely plot standpoint, it is also relatively dependent on the genre you're writing. People return to certain genres/sub-genres because (and these exist in the first place because) people enjoy having certain specific emotions evoked. Like Sci-Fi/Fantasy = Wonderment and Immersement. Crime/Mystery = that feeling that the reader can "figure it out first," or "see it coming."
So readers of certain "popular fiction" genres generally expect (and enjoy) a high complexity of plot:
And the expectation is often for less complexity of plot (as the focus is elsewhere) in:
But these are, of course, not hard-fast rules. And as for literary fiction, the umbrella is just too large to really have any solid answer one way or the other.