Cotton batting will hold a fold crease more than any other fiber. The only way to completely eliminate the fold lines is to saturate the quilt and block it or steam it but blocking will do a better job and also get the quilt nice and flat and straight for hanging in the show. You will then have to leave it flat until the show as any folds will create crease marks and new fold lines. This will leave the appearance of an unwashed quilt.
The other option is to steam the quilt with a clothing steamer. As you have to hold the steam head to the surface for several seconds, it is not advisable to do this with an iron as you will quickly fatigue given the small surface area you are hitting with the iron's steam function. And the fact you need to get the steam through all 3 layers of the quilt, but especially the batting because that is what is causing the fold line to be so obvious.
Only moisture, in the form of wetting the quilt or steaming the quilt will get rid of the fold lines entirely.
Fold lines are a real problem for show quilters who ship their quilts to the big shows. Because the cotton holds on to those creases just from shipping. Have talked to so many frustrated show quilters when they see their quilt hanging in the show with massive fold creases from the shipping or even from show handling. Have chatted with quilters who rolled their quilt around a core to avoid the creases and the quilt still gets them from the show folding them between judging and hanging.