Is ordinary an noun

Generally ordinary would be an adjective, not a noun.

You could possibly use it as a noun if you started using it a class - sort of like in the Potterverse you have the term "muggle" for non- wizadring folk. You sometimes will see authors use it it that way - especially when creating their own worlds, e.g.

"Gylla was an Ordinary but she had many friends who were Crosses."