Rankings can tell you how fashion schools are viewed in the academic community, and give you an overall idea of the success of graduates and school reputation, which is very important in the fashion world. The status of the fashion school you go to may weigh heavily down the road with an employer or client, so that element of rankings should certainly not be ignored.
But even when you compare multiple rankings (keeping in mind the criteria that was used to formulate them), that comparison should only be the beginning of a thorough college search. The best college decision will be determined by the factors that are the most important to you, such as cost, location, family alma-mater, and program emphases, among others-features that rankings scales usually don't (or can't possibly) consider. So don't lean too heavily on what rankings tell you. Other factors determine great fashion schools.