I think it may help her even with people who don't watch the interview. Just hearing that she was willing to go on Fox may make a favorable impression since for years now, neither side has bothered engaging the other side by agreeing to an interview by the hostile network. It's the opposite of Hillary's "deplorables" comment. It signals that you want to be president for all Americans. It signals that you have the courage of your own convictions. It showed that she can hold her own in an unfriendly environment (women have to prove that they're tough; men enjoy the assumption that they are). In correcting Baier, she got to say things that right-siloed people might not previously even have heard about Trump. It's unlikely, in itself, to have actually won her a vote, but neither will it have lost her one, and it may add in an incremental way to people's positive impressions of her.
She can, for rest of the campaign call Trump a coward, both for dodging a second debate with her and for not being willing to submit himself to such hostile questioning.
Edit: Remorseless made some of the same points while I was typing.