The general rule is that a person’s behavior does not invalidate that person’s arguments. This is because ideally philosophical arguments are based on reason. Thus if the reasoning is valid, it does not matter whether the person offering the argument can successfully do what they believe is the correct course of action. Thus, several of the other answers speak of the hypocrisy fallacy.
This fallacy, like most (with the exceptions being fallacies of formal logic like affirming the consequent and denying the antecedent), are not however absolute nor are they strict. By saying they are not strict, I mean merely being able to point out that something can be called a fallacy does not mean that you are not identifying a valid problem with an argument. By saying they are not absolute, I mean that they involve a human judgment as to whether or not the fallacy fits the argument well.
For moral arguments, I think there are some cases where it does matter that the person or group offering the argument is unable to do what they tell others to do. Here, the material point is below the surface of the argument and hinges on questions of whether "ought implies can" (as Kant famously believed). The point being that some failures to execute on a moral argument may indicate that the reasons and premises used are somehow incompatible with the human condition. Again, whether this matters is a question of what type of moral philosophy one practices -- specifically on whether it believes we can be moral (which is different from being a realist about morality).
To give an example of one area where consistency can matter, Plato condemns homosexual sex in the voice of Socrates, but he also writes that Socrates lusts after some of the male youth and indicates that he had slept with them. Given this sort of dilemma between one's moral rules and one's actual practices, one can choose either to change the rules, change the practices, or admit that one does not achieve morality.
A second type of problem can also make this relevant. If we are looking in terms of moral examplars, then it can matter if they person who tells us we must always behave in a certain way does the opposite herself. Here, the point would be that we take certain figures to be moral authorities by virtue of their virtue or some station. I take it this is one motive in the anger towards the Catholic church about priests abusing children (when in fact statistically, they are not more likely to abuse than the general population).
All of this to say you can call it hypocrisy but that only becomes damning when you combine that with other background questions about the possibility of living morally and the nature of our relationship to our moral experience and about how we see certain parties as moral advisors.
Returning to your specific example about the hypocritical environmentalist, it might matter if the environmentalist wants his listener to feel guilt over polluting yet expresses no remorse himself. It could also matter if he claims this is an absolute good -- but demonstrates in his own life that this is relative good. But it will take more to flesh out why one thinks these claims matter.