There is nothing wrong in complimenting a female OR male, as long as the compliment is said sincerely, and not too effusively. In the case of the girl, she probably took a lot of trouble to look good for her date. It’s good positive reinforcement to let her know her efforts were not in vain. I disagree with Obaminator.
As to the rain issue, get a great umbrella and terrific rain coat with a hood, and have fun. Rain can be very enjoyable, as long as it is not a hurricane. I don’t recommend an outdoor picnic, though.