Environmentalism was increasingly in the American consciousness in the late '60s and '70s, but this song isn't actually about that, despite the title.
"Mother Nature" is about yearning for freedom from modernity and seeking a more natural state of living. The words express spiritual dissatisfaction with civilization.
Day in, day out I learn to find
Leaves me half alive
And I'm going nowhere
I can't live in this classic world
It's killing me
Can't you see
You're the only one to turn to
It was common for popular musicians to romanticize nature in this period of popular music, which carried the calling cards of the '60s hippies but evolved into something new stylistically and thematically. In many ways, this song is perfectly illustrative of those changes, in fact.
The '60s counterculture started as a pointedly political movement concerned with civil rights, the anti-war movement, and equality. Over time it became a cultural revolution first and foremost, with politics more on the periphery. Then, after the whole thing started to fall apart in 1969, the '60s revolutionary sentiment was turned decidedly inward into "self-empowerment."
At this time, people became more concerned with changing themselves than with changing the world. That concern is essentially what's expressed in "Mother Nature," though all the way back in 1972 it's unlikely that the writers had a broader view of the changes happening to society at large. They were simply speaking as part of it.