Solar panel output is remarkably resilient to visible dirt. I have an installation on a seagull rookery. As you can imagine things get thoroughly “whitewashed”, especially since the top edge of the angled array is prime “standing around squawking and crapping” space. They can be covered to the point that there isn’t a clean spot the size of your fist anywhere on them and they will still be putting out 75% of their rated power.

I suspect it has something to do with the non-linear response of the human eye. You don’t have to reflect a lot of light to be perceived as significantly lighter than black, so diverting 25% of the light from the panel to reflected light makes the panel look quite light.