Matching stripes horizontally is a challenge. Here's what I do:

I offset the pleat that I'm stitching to (the new one) a little to the right relative the the existing pleat (maybe 1/32") when I pin, because stitching tends to generate exactly the kind of mismatch that you show in your pic (next pleat stair steps to the left), so offsetting it the other way when you pin helps to compensate. Then, when you stitch, be sure that you bring the needle up exactly at a color boundary when you get to it and poke it down exactly at a color boundary. With a little practice, the combination of the two solves the problem most of the time. I still take out and re-do probably one pleat in every kilt I make because it has a tiny stair step.

Barb