People follow and unfollow all the time, and this author probably won't know the unfollow was even from you. Unless they had some sort of ad-on that could track that (which I don't think exists for Tapas, as far as I know) It would be kind of hard to figure out. They'd have to sleuth through their entire list. Besides, the act of you unfollowing them is explanation enough. They don't need any more explanation (and probably don't want one).
But the thing about webcomics is that they're so fluid, they often go in several directions as they're updated--it's a constantly changing story, and as the author grows up and matures, they may decide to take it a different way than they originally intended. Personally, if I went through the effort to follow a comic, I rarely unfollow unless they do something that straight up really bothers me. In a few months time the comic will be completely different all over again, and who am I to say at all where it all might be going?