It depends. There are certain hairstyles that might really interfere with my attraction to someone. Others I'm just not crazy about, and I will still be attracted to them.
Hair is a very changeable thing. If it seems like his hair really reflects him and is part of him and I don't like the style, then that will be something to think about. If it seems like he just made a bad style choice, then it won't be a big deal to me and I'll just overlook it or hope that he gets a better style in the future.