How would you describe stained glass as art?

Yes, it is correct to talk about the art of stained glass. It matches the criteria, just like any other artistic medium (e.g. painting, sculpture, printmaking),of requiring skill and imagination in its production. However, the people who undertake the work of translating the design of a stained glass artist onto glass are generally called (master) craftsmen.
For instance, Marc Chagall designed windows depicting the 12 tribes of Israel for the Hadassah Medical School, Israel, but the work was carried out by Charles Marq, a craftsman in a famous workshop in Rheims, France.
See pics http://www.md.huji.ac.il/chagall/chagall.html