The Japanese have a term for something that appears simple but is actually very complex in detail: Shibui. It should be said that this is only one aspect of Shibui, as with many Japanese words/concepts.
Is there a term for the opposite: something that appears hopelessly complicated, but in actuality is quite simple, whether this is because it follows a subjectively unknown pattern, or because of the subjective perspective of the person?
In OP's context, everything is either simple or complex, and for any given thing, either it's obvious which applies, or the appearance is deceptive.
Things that both appear to be, and really are, simple are hardly worth mentioning. Nor is it normally worth saying that something which appears complex really is complex. So the only cases worth mentioning are things that look simple but are actually complex, and things that look complex but are actually simple.
As others have noted, the first case is often called deceptively simple. We often admire things which are superficially simple, but which we know are actually complex, so it's effectively a complimentary expression - which once given is often followed by an explanation/discussion of the "hidden" complexity.
But we don't admire complexity for it's own sake - quite the reverse, in fact. If you're giving a value judgement on something meeting that criterion, you'd probably say something more scathing than deceptively complex - which barely exists by comparison with the "standard" version (I think people avoid the inversion partly because it might be misconstrued as a compliment).
In practice the context where we need to say something is simpler than it appears is often when we're speaking to someone else who is confused by the superficial complexity. In which case we reassure them by saying something like "It's easy, really". And then proceed to explain it.
Just because there are four positions in the "truth table" under discussion, doesn't mean we need a "matching set" of four names. Two are just trivial. The other two normally arise in different contexts (though in both cases it's likely you'll carry on by actually explaining whatever it is).
EDIT: The nearest "real-world" antonym I can think of to something that's deceptively simple is emergent complexity, a term often used of systems wherein a small number of simple elements are repeatedly recombined to create something complex. Particle physics, fractal designs, or living organisms, for example.
We call a thing which is actually simple but appears complex deceptively complex. Both OALD and CALD call something which is actually complex but appears simple deceptively simple.
A 'ruse' is meant to deceive, to hide the truth, possibly by making something appear more complex than it is. Ruse does not explicitly relate to complexity vs. simplicity, but it does relate to the subjective perspective of the person: they are being deceived by a trick that can easily be exposed, thus revealing the truth. And truth is often simpler than deception.
The only term that comes immediately to my mind is "deceptively simple".
As an example, if I ask you to tell me the area of a triangle with sides 4, 7, and 11, you might go and dig up Heron's Formula (area=sqrt(s(s-a)(s-b)(s-c)), where s=(a+b+c)/2) and start calculating it, or you might notice that side a + side b = side c, and thus the "triangle" is actually 2 straight lines, and therefore the area must be 0.
"Something that appears hopelessly complicated, but in actuality is quite simple" might be called a Gordian knot, of which wikipedia says,
The Gordian Knot is a legend of Phrygian Gordium associated with Alexander the Great. It is often used as a metaphor for an intractable problem solved easily by cheating or "thinking outside the box" ("cutting the Gordian knot")
Note, this answer was suggested by answers to a related question, which apparently seeks the four-word phrase that the wikipedia quote shows in parentheses, or perhaps "thinking outside the box", or perhaps something else entirely.
