Dust on the solar panels is a major issue for Mars landers without an RTG, and InSight does have a robotic arm. So adding a brush of some sort (either fixed to the arm or graspable) sounds like a good idea. It hasn't been done and (AFAIK) no other lander had this either, ever. What could be the reason for not including a brush?
Using a brush with thick, pointy bristles could scratch and damage the sensitive surfaces of your solar panels as you scrub. The same kind of brush that you use to clean windows on your home or car should work here. If at all possible, try to get a brush with a built-in water-fed system.
When picking a brush for solar panel upkeep, make sure you grab one with soft, gentle bristles. Using a brush with thick, pointy bristles could scratch and damage the sensitive surfaces of your solar panels as you scrub. The same kind of brush that you use to clean windows on your home or car should work here.
– SF. Reasons not to provide mechanical means to clean solar panels on Mars: and this is the primary reason: Wind on Mars occasionally blows the dust away. This means dust is not a major issue, but a minor one. Spirit and Opportunity functioned for years despite not having dust removal equipment.
NASA is looking into technologies that can clean a solar panel. This has been identified as an enabling technology for crewed missions to Mars. Evidently, NASA does not see this as a necessity for robotic missions up to and including InSight. Electrostatic cleaning: this leverages the static charge of dust particles.
While regular washing is needed to keep panels clean, it's not advisable to use water and soap. The residue or film that sticks to the panel after using soap can attract dust and create dirt buildup faster. As a result, this affects the efficiency and output of the panels.