3000 day/night cycles, which are possibly more wearing on the equipment than the sheer passage of time.

Probably less so for Curiosity than the other Martian rovers. It's powered by an RTG rather than solar panels, so the only difference between day and night is the temperature fluctuations, which is still significant.

Do light fluctuations cause wear on solar panels?

Panels wear slowly, but just from being hit by photons, blasted by dust, etc. - but that’s not the problem - batteries are. They don’t last forever, and every cycle takes a little out of their lifetime.

Not really, but battery cycles and powering things on and off constantly does take a toll.