Acertain material has a density of 0.2 g/ml is this material good for building a bridge or for making cushions for a couch

A bridge must be made of metal or concrete or other materials that are strong enough to withstand the forces of the traffic passing over it. I doubt that the material could do that.

If the substance is a metal foam, it would make a rather hard cushion.

If the substance is too soft, it would make a cushion.

If the substance has the right combination of softness and elasticity so that it returns to its original shape when you get up, it might be a good material for a cushion.