Learn English – Term for something that is supposed to increase safety, but really just increases fear

Orthogonal

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.