Why not? If you have a unique hairstyle and want to commercialize it, you can indeed trademark it with the patent and trademark office. A hairstyle is a human creation and like most creations, it can be an asset, and in this case, it is an intellectual property. If it gets a trademark, other people will have to get your permission just to copy it. However, you will be required to officially file it with the patent and trademark office first and pay the official fees and do the necessary paper work for the filing. Of course a trademark needs a name, so you will have to come up with one, as unique as possible, so that the trademark office will not have reasons to reject it. But for all intents, trade marking is a means to prevent others from using the same idea and form whether for own or commercial purposes.
Whether you will be allowed to trademark it will however depend on the trademark office, but there is a very good chance that it will be allowed. Now, in order to be allowed for a trademark, its form has to be unique, something that can stand out from the crowd, so that people can identify it as different from the norms. Like company logos, your particular hair style got to be reproducible and it has also to be identifiable by a name. For the purposes of trade, the form and name will be easily identified so that others will be able to communicate to others about it.
A hairstyle need not only consist of hair alone, for it is also possible to weave it with some other items like hair bands and bangles. In fact you can make it unique looking with a whole range of hair accessories or with something that you design yourself. And remember, if it is new and nobody has it, you can file it and make it your own. After which you can commercialize it and license other salons to recreate that particular hairstyle. Of course, you will need to make it so desirable that others would want to copy it and pay you a fee for doing so.
And thinking about it, perhaps those people who had been promoting fashion shows should have filed for trademarks for all the lasses who had sported unique hairdos as part of the fashion presentations!
But what if someone copies it without paying you for the idea? Well, getting a trademark is one thing, enforcing it is another thing altogether! The onus is that you will have to report any form of infringements and to institute legal action yourself! But even if you don't get a cent for the idea and supposing that the particular hairstyle becomes a hit, just people talking about it will inevitably boost the value of your idea and your trademark. If it is a buzz, you will benefit from it many times over. Also, that trademark need not be associated with only a hairstyle as you can use it for other things as well, provided of course you also register it under other item categories with the trademark office. The opportunity is limitless as we are not only talking about human beings, but also others like pets and anything that needs a unique dressing!