Install it as per its instructions and then go to its settings panel. YITH > Wishlist > Add to Wishlist options tab (at the top).
You will need to change it to the following settings.
Set “Add to wishlist” icon to None. And the “Added to wishlist” icon to Same used for Add to wishlist.
Why are you doing this?
It prevents a double heart icon from appearing. And ensures that YITH doesn’t load the entire Font Awesome icon library, slowing down your site, all for the sake of one icon.