I think you have to put yourself in the clients shoes. If you were awaiting a mobile therpaist to come to your house to spray tan you, and you saw her walking down the drive with a uniform on - you would automatically see her as a professional who takes her role seriously and is keen to give off the right impression.
Yes it is important to be comfortable, but I think no matter what you do within the beauty industry its important to be smart, clean and presentable. It also displays your comittment to your role.
Some might say that a uniform and looking perfect can make clients feel intimidated or uneasy but its your personality that will ease your client and make them feel comfortable having the treatment.