Why do my zig zag or decorative stitches crumple the fabric?

You need to support the fabric with some stabiliser, (Stitch n' Tear or ordinary typing paper work well to start with) so that when the machine forms the stitch the thread is not able to drag the the fabric upwards. A zig zag foot with more surface area underneath is helpful here but a satin stitch foot is easier to see your stitches and has more room for any bulk created with your thread so a choice has to be made.