I'm guessing it's because the AC voltage will introduce noise on the DC low voltage current.

But like everything else it depends. Is the DC feeding something like a lightbulb or a fan? Then it probably won't matter.

Does the DC feed something that has a good low pass filter or is it an expensive AC cable with good shielding? Then it might also not matter.

None of the above should be considered be professional advice and it's probably a good rule of thumb to never mix wires like that.