It is if it makes someone reject the argument. This is identical to saying "Climate change doesn't exist because Al Gore flies in airplanes," or "Ralph Nader c

It is if it makes someone reject the argument. This is identical to saying "Climate change doesn't exist because Al Gore flies in airplanes," or "Ralph Nader cannot be a critic of capitalism because he made a million dollars of his books." Motivations have nothing to do with it, that is exactly what an ad hominem attack is. If you reject the argument because of the person's motivations, you are calling an argument false without evaluating the argument itself. That is broken reasoning, which is why the term "fallacy" exists.