For an experimental helicopter that must fly around, Ingenuity's mass budget is extremely tight. I don't know how they'd put something on the helicopter, and I imagine it's considered far too risky to have the rover have some way of cleaning the helicopter.
Or just hope the vehicle's flight envelope is wider than the "dust's ability to stick to the panel" envelope. You're gonna lose the vehicle either way so why not try it.
I think part of the problem is that the kind of people who have a lot of experience solving these kinds of problems don't really have career paths that intersect with NASA so the engineers spec'ing out the vehicle don't really know with good enough levels of certainty when they can make cheap minor tweaks to the design of the vehicle that will produce some secondary positive result. And you can't go just call up the professional specialists and ask for their input because the processes involved are too bloated and would cost too much money to invoke over small secondary features so you really need that kind of expertise in house or not at all.