Visual observations of Mars are made when it is at opposition, when the Sun, Earth and Mars are in a straight line and the distance to the planet is minimal. Unfortunately, this doesn't happen at the same season (on Mars OR Earth), so the observations would not really be comparable. Every opposition occurs at a different point in Mars' orbit. Oppositions are pretty infrequent too, every few years as I recall. Also, since the visual appearance of the polar caps is tiny, even with the biggest scopes, it would be difficult to make observations of sufficient precision to be of value. Photography never worked very well on Mars, either, since the planet's rotation blurred the time exposures needed to show detail on film. We knew there were caps, and that they shrank in summer, but that was about it.

Another problem is that Mars' orbit is more eccentric than Earth's, so it's distance from the Sun varies much more, percentagewise, than Earth's. Any solar-driven variation in icecap size would be masked by the geometrical effect on insolation.