A light bulb is non-ohmic because it does not obey Ohm’s law. According to Ohm's law, "The ratio of potential difference to the current flowing through a conductor is constant, providing all other influences such as temperature are kept constant."
When a material is ohmic, the resistance remains constant when the temperature is kept constant. Most materials are non-ohmic because they do not remain constant if the temperature is kept constant. Light bulb filament resistance depends on applied current and voltage rather than a constant temperature. Ohmic materials are mainly metals and usually apply to a limited range of currents.