A "general misunderstanding" isn't really a theory whose complexity you can compare to the Dark Matter theory.

One could also argue that on the one hand you need just one concept (dark matter) to explain three phenomena (galaxy rotation curves, gravitational lensing, CMBR), and the "general misunderstanding" might need to have three aspects to make it explain all three things, making it more complex.

Occam's razor is a good guiding principle, but in some cases, it's really hard to compare the complexity of competing theories. My favorite example of this is the Many Worlds interpretation of quantum mechanics. In some sense, it's really simple, the wave function just follows the Schrödinger equation. In another sense, it implicitly postulates the existence of many parallel worlds, which you can count as a quasi-infinite complexity penalty, or you can shrug and say it's not a big deal, the theory says the parallel worlds aren't observable, so there is nothing wrong with postulating them. There's no objective way to decide that.