No reason why not, although the green shade net might be more effective. The plants won't mind the heat but they will be affected by any loss of good light, so shading is okay at the hottest times of day, but best not for the bulk of the daylight hours. Rig up something which can be pulled on and off?
However, far better is to roll up the sides of the polytunnel, open the doors either end and get an airflow in and through, cool it that way?

We have a small polytunnel and as Worthy has suggested have the doors rolled up at both ends and the side openings rolled up too. Seems to be working okay for us but it is probably hotter where you are than it is here. Not sure hosing down would be a good idea in any case as this will simply increase the humidity in the tunnel.

If its easier maybe you could rig up some shade netting inside the tunnel, depending how big it is of course

Thanks guys yea it's a big one I leave the doors open and give a good watering in the morning but the last few days I've had some very sad plants :( not complaining tho I love this weather it doesn't help that's its in full blast of the sun from morning till dusk the whole garden is apart from my patio where we sit oh the irony I think I will contact first tunnels for some shade net and give it a go