it's hard to judge w/o watching you sew. but, from personal experience when i first started quilting, using strips, i found that i tended to pull slightly on the strips as i sewed resulting in wavy sewn strips as in your picture. while teaching my grand daughter to sew on machine i've had her concentrate on going moderate even speed with fingers guiding, not holding, the fabric strips with very light pressure.
your border looks like it was too long and then pressed which made it wavy. i measure my quilts across the middle vertically & horizontally ..pin border strip at corners, center, then center again and again til pinned. i also know that many quilters are not the pinners that i am. but i learned to sew via clothing & household items like draperies etc ..pinning is fitting and i continue the method with my quilt assembling.