Let me just say one thing first: I love Grease (1978). Always have, always will.
The style in in this film is actually quite amazing. Everybody looks fantastic, especially Sandy.
Sandy's style change from good-girl to bad-girl is awesome, but I have one major problem with it: she changed her style for the entirely wrong reason. She dropped her look because she wanted to impress a guy.
The fact that the look that she went for was better than her original look, is entirely beside the point. Never change your look or style for anyone, especially an idiot.