“Dress For Success” is a saying you hear bandied about, invented who knows when. I’ve found it accurate for interviews, but not everyone applies the same principles. I find that, especially at conventions, a lot of people don’t realize the impression they put forth when they are trying to sell books, etc.
If you are an author, artist, etc, conventions are part of your job. Dress the part. Smile for photographs.
The elusive “unphotogenic author” is a myth that authors have invented to excuse a pervading fear of people rising across the nation.
Impeccable clothing (or a good costume) can be considered armor. If you are afraid of people, dress well. Talk to them before they talk to you. Control the situation. Make eye contact.
It was not my intention for this miniature bit of advice to be a rant, but there you have it.
Those who make friends from the people who pass them by sell more books. Or action figures. Or caricatures. Or whatever.