Puckered knitting

I lengthened a sweater by adding a boarder. I did the boarder in garter stitch. The body of the garment is knitted in a shadow cable stitch.
The problem is that the boarder puckers - looks wavy . Did I pick up too many stitches? Or perhaps my guage is wrong…or both…
I would like it to lay flat any help would be truely appreciated.

When picking up from a cast on edge you would pick up a stitch in every st along the edge. It may be that the cable st pulls in a little and garter stitch is a little wide over the same number of stitches, so you might have needed to pick up less, but going down a needle size would also work. Don’t do both a smaller needle and less stitches though. I think using a smaller needle would be better because skipping stitches on the edge would look odd.

Okay…I’ll go down a needle size. Thanks