I do not recall our Lord ever saying one could not be 'cool'. It is only a problem if one esteems 'coolness' above that which is righteous and true, which is, when we give it its way, really what many of us do. 'Coolness' is too transient to be of any real and meaningful, lasting significance, and it is often in great conflict with one being one's most honest, most vulnerable self. That, and in reality, some of the coolest people are actually those who least concern themselves with being cool anyway, those who make 'trying to be cool' less evident.