There's a little trick to getting a sewn-down outer pleat to lie falt and it depends on the nature of the fabric. When I was making the prototype X-Kilts, I discovered that the cotton duck-canvas kilts lay pretty flat at the pleats, it was the cotton/poly twill that curled. The way I discovered (totally by accident) to help with that was to put the sewn edge under a little bit of tension when it's being sewn. In other words, pull it through the presser foot/feed dogs while you're sewing it. Somehow that little bit of tension...not a lot, just a little, helped with getting the pleat edges to lie flatter.

Why it does that, I have no idea.