I don't think it's weird that a child of that age wants to dress in a girl's costume. However, what I do think is odd is the number of adults who think it's ok to allow a child to do whatever makes him feel good in public. There is a proper way to dress and behave in public, and it's my job as a parent to teach my children proper manners, etiquette, dress and conduct. My children are not allowed to do whatever makes them feel good in public. They understand that they are expected to be polite (even when they don't feel like it) to dress with modesty (even when my 14 year old wants to expose her midrif) to not stare at people, to not play in public fountains in their underwear (even though it would just plain feel good)
With that being said, the rules are different in the privacy of our home. My youngest son, when he was 3, loved his big sisters' costumes so much that he loved dressing up at home in their costumes, carry their purses and wear their high heels. He is a beautiful child, and at 3, he had chin length curly hair that his sisters often teased into a pony-tail. We loved playing dress up and his dad and I took many pictures of him all "girlified".
Now, if my son at that age wanted to dress up that way in public, (which he never did) I would impart some wisdom and explain that there is a proper way to dress and behave in public. Though I would not judge a child who is inappropriately dressed, I would not be able to help myself but to think that the child's parent is a little "different". If that makes me narrow-minded, then so be it.