It depends on what you mean by professional, and what type of job you're applying for. If you mean, do you have to wear a suit to every interview, then no. Follow the lead of the interviewer on that one. If you mean, do you need to present yourself well at that interview to prove that you're serious about doing a good job and being an asset to the company, then yes - absolutely!
I know one person who presented herself very well at her latest interview - knowledge, appearance, demeanor, and so on - until she mentioned that she was crazy. She explained that her sense of humor was crazy, but that she was completely focused when it came to her performance on the job. It was too late. The words "I'm crazy" stuck in the interviewer's head, and she didn't get the job.