It was Hillary Clinton in her campaign who, as far as i know, was the first to use this term.

The term predates that. I recall seeing an interview of self-described alt-rightists on YouTube well before either party had selected their candidate; I had heard the term bouncing around before that, and watched the video to try to understand what it was about.

But, yes, once it became clear that the alt-right definitely had some unpleasant traits, the Clinton campaign was glad to use it as a smear.