Would "Це спідниця - модна але дуже дорогa" not be kinda correct?
No, because спідниця is feminine and needs the feminine ця.
why is the "it is" translated as "це" and not "Ця" to go with all the other feminine descriptors?
Because "it is" always translates as the neuter це. Ця means "this" when describing a feminine noun, like ця спідниця, "this skirt." When you say "it is" or "this is," the translation is це.