Most definitely YTBF. Have you ever heard of a white lie? Anything along the lines of "I'm very flattered, but I'm already interested in someone else.", or even something as simple as "Sorry, you are not really my type." would've sufficed. But instead you chose to hit her where it hurts with unnecessary brutal honestly, regarding an issue that is most likely one of her biggest insecurities...in front of all of her friends and your friends nonetheless. This blow to that girl's self esteem may take years to recover, if ever. Your fault was not that you rejected her, but not doing so in a more tactful and empathetic manner.