In this recession, the government needs every source of revenue it can get. People who are wealthy enough to afford elective surgery will also be able to pay a tax on it. This could be an important new source of money for the government. A tax could also deter people from getting unnecessary surgery, which is a little perverse.
I've had multiple surgeries (not plastic--for other medical conditions) and when one of my incisions' recoveries didn't go well due to an allergic reaction, I needed to have plastic surgery. The incision literally stuck to my bone, and since I was young at the time the bone grew and my skin stretched with it. So for me, the surgery was necessary and adding an unnecessary tax to a necessary surgery is cruel. I also know plenty of other people who've needed cosmetic surgery in order to deal with their medical conditions, so this surgery is not unnecessary to some.
Although, these surgeries are not necessary, they should not be taxed extra for wanting to improve their looks or body. The surgeries are often done to improve self-esteem and mental health. Those benefits are also great for a person and they should not be taxed extra for wanting to improve their mental health but improving their physical self. There are specialty doctors for this, it is not as if we have a lack of doctors for this.