Dear student!
Actually, the %age of carbon is low in atmosphere in form of gas only like CO2 etc. But,on the earth, carbon is highly abundant as it is found in all the organic compounds in the form of hydrocarbons starting from a plant to a human body everywhere carbon and its compound are applied. Even our fat, protein , cellulose etc. are all carbon containing compounds. Not only as compound but carbon is in natural state too in the form of coal and natural gases petroleum etc.
Though, the content of carbon is less in comparison to other material because,the organic sources are not worth extraction which reduces is practical percentage appearance by mass.