It is sad but absolutely true: appearance does matter, especially when you are applying for a job that values so much a polished image.
In terms of birth marks, the rule of thumb is that if it is bigger than a coin, then you have to declare it when asked about it. It will be noticeable anyway during your medicals and might even put you in danger to be sent home.
You can maximize your chances by having a good image during the interview.
If there is a treatment to minimize the appearance of the mark and you are willing to do it, then by all means you should consider it!
Good luck!
