A picture would definitely help. But I also have a couple questions.

-Is the ripple in the pleats or the apron? You don't mention which. I'm having a hard time visualizing how variations in pleat size could give you a horizontal ripple.

-One of the things that can give this kind of horizontal ripple in either the pleats or the apron is buckling the kilt too tight at the hips. The hip strap should be loose - it does nothing to hold the kilt on. The only function of a hip strap on a dancer's kilt is to control the apron and keep it from flying around when the person is dancing. Many people yank on the hip strap until it it really tight, and that can distort both the apron and the pleats. Have you tried pressing to see if the ripple will press out?

And do you have any idea on the extent of canvas in the kilt, particularly in the pleats? The purpose of the canvas is to allow flexibility around the person and provide stiffness vertically. If the canvas is skimpy, or if it's been put in with the stiff direction the wrong way, both the apron and the pleats can develop horizontal "waves" if the kilt is buckled too tight at the hips. Whether this can be pressed out depends on whether the canvas has "locked in" the distortion.

Anyway, some pics would help.