Can you cast Dispel Magic vs a hidden (invisible) creature?
So long as the creature is hidden and not undetected or unnoticed, I believe you could do so.
When targeting a hidden creature, before you roll to
determine your effect, you must attempt a DC 11 flat
check. If you fail, you don’t affect the creature, though the
actions you used are still expended—as well as any spell
slots, costs, and other resources. You remain flat-footed to
the creature, whether you successfully target it or not.
So targeting a hidden creature with Dispel Magic would still require succeeding on a DC 11 flat check to effect them, but assuming you did so, they are a valid target for Dispel Magic.