There were a lot of eccentrics of various stripes back then. Civil disobedience is an American tradition (that Thoreau codified). There is nothing wrong with it. Have you actually read Walden? Thoreau speaks admiringly of a train heard nearby. He’s no more to blame for contemporary Environmentalism than Nietzsche is for contemporary nihilism.
All depends on what you're being disobedient about.
I'm not sure Thoreau causes were noble.
I have not read every word of Walden Pond, but yes, 25 years ago at the Evergreen State College, we read Walden Pond and you are the first person that I have ever heard suggest that modern environmentalism has little or no connection to Thoreau. Thoreau is the absolte poster boy of environmentalism.
While many modern environmentalists paint Thoreau as an anti-government libertarian free spirit, it must be pointed out that the American government of his day was very limited in its powers. It also openly espoused individual liberty and Manifest Destiny. In other words, to be a rebel against the limited constitutional powers of the American government suggests that Thoreau was not the libertarian or free individualist that many often make him out to be. He was protesting/rebelling against a limited government based on liberty and Judeo-Christian values.