One problem might be when she comes to iron the top. The jelly roll being cotton, will need a hot iron, but that might be too hot for the poly cotton, which could shrink with too much heat. I really like to only use 100% cotton, but given the lockdown and difficulty getting things , perhaps needs must.
Having said that I recently ordered 100% cotton bed linen online in a 70% off sale to use as backing. It arrived very quickly and worked out much cheaper than buying fabric.