Is Indian corn used for decoration only?

No, it is not. But its not to common. Indian corn is a starch which is edible but probably tastes bland. Corn that people normally eat is juicy and sweet while if you were to have it before it matures from the internal breakdowns (while its green), it would be hard, crunchy, and bland. Likewise, Indian corn is hard and bland but its not commonly eaten. If you were to use it, it might be best to ground it into a form of corn starch and mix it with something else.