I once heard that when you prepare for shabbat meals and holidays----you should go to extra effort to make it especially beautiful---e.g. your table settings, your clothing that you try to make it beautiful, not just ordinary every-day. Is this as mitzvah or just a minhag?

There is a special Mitzva to respect and honor the Shabbat. This is learned from the Pasuk: Vechibadeto Me'asot Derachecha. The Shulchan Aruch writes that this means to honor the Shabbat with nice dishes and clean clothes. Rabbi Yitzchak Grinblat