Radicalism is mostly supported by those who already support a cause. People who are unaware of a cause, or not in support will typically dismiss the message of a radical performance and focus on rejecting the behavior. The message gets lost, and the only people that cheer are already on your side.
No one is unaware of climate change as a topic. The rate, severity, and urgency of climate change are what gets consistently misrepresented and suppressed. Those are the points that need to be communicated far and wide, and I don’t see how painting Stonehenge compels anyone to learn more about it or join future protests.