Is "environment activist" a proper noun? Should it be capitalized?
An "environment activist" or as I've usually seen it "environmental activist" is a common noun - much like the name of any profession or occupation (ex. student).
However, if you were to use the term to name a specific individual, then it'd be a proper noun. Like this:
"In my classroom, we have the Lawyer (always negotiates with the teacher about what should be on the test), the Jock (captain of every sports team in school), and the Environmental Activist (insists that Biology should be called Life Studies).