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.
Would "Це спідниця - модна але дуже дорогa" not be kinda correct?
No, because спідниця is feminine and needs the feminine ця.
Really getting annoyed at the commas that are thrown in but not in the original translation