IIRC, LLF once pointed out that you can't really sit in real versions of dresses like that.  I think it was her, that is, I'm sure of the quote.  The person, who was speaking from experience,  said in real life people have to give you a stool sit on under the dress.

You can just make ruffles like those by extruding out, using a select every other n, and pulling in the appropriate direction.  If you need extra edge loops, cut one close to the join on the ruffles, then at least one before the end (so the ruffle has at least two internal loops with one close to the join). Select all the loops but the join and the loop next to it, then subdivide them as many times as you need.  I usually find no more than 3 works well, but you might need more for that topology.  Then use J to make the n-gons between the last subdivided divided loop and the loop before the join into quads and tris.  The tris will actually help with the rippling effect.