Many people don't hesitate using different weight fabrics, especially for a wall hanging. Sometimes it is the color or pattern or texture or something that speaks to us.
Dunster gives some good reasons for not using it as a binding, but you also mentioned border -- which I think would be fine. Prewash and maybe pink the duck cloth if it seems to want to ravel.
Earlier today we were talking about muslin and cotton, same thing with Duck it mostly it is a weave difference and then weight differences. Duck is basically a canvas, it can be all cotton or a blend. Kettle Cloth is a bit different than duck but was used along side and with flour sacks for use quilts despite the texture difference.