It is up to you to decide what kind of stitching (or fusible) to use when applying the appliqué to fabric. You might want to “show off” your stitching by using a decorative stitch. You might decide that you want to hide your stitching by using an invisible stitch (or appliqué stitch).
In this photo, I have used blue thread on some of the stitches to show the invisible stitches. To the left of the blue thread are invisible stitches made with matching thread.