Can a crocheted poncho become a skirt? Yes! For instance, if your poncho is two rectangles joined together, then instead of stitching a neckline, you simply make the adjustment for a waistline.
You add elastic to the waist in one of three ways. You can fold over the top to make a casing to slide the elastic through. OR, you can sew a fabric casing to the inside of the waist and slide the elastic through it. You could even make large stitches in an X pattern to hold the elastic in place on the inside of the waistline.
Don't want to hastle with elastic? Use a drawstring instead.
As long as the poncho will fit the waist, it can be a skirt.