Light bulb filaments are made of tungsten. Tungsten is a metal. Metals are
ohmic substances. Ohmic substances obey Ohm's law. What it *means* to obey
Ohm's law is that for a uniform current through a uniform sample of the
substance, the current intensity in an asymptotic steady state is
proportional to the electric potential difference across the sample for a
wide range of current intensities and potential drops. The proportionality
constant *may* be (and almost always is to some degree) temperature-
dependent. The proportionality condition is to hold for a given *fixed*
temperature of the sample.
If the temperature of a light bulb filament is held constant by an
appropriate thermostat then for any temperature you pick for it to operate
at (assuming the it operates below the melting point of tungsten so it can
hold together) you will observe that the current through the filament is
directly proportional to the potential difference across it over a wide
range of currents and potential differences. Therefore at each temperature
the filament has a well-defined ohmic resistance. Just because this
resistance is temperature-dependent is not an excuse to claim that light
bulbs are not ohmic. After all, all realizable resistors have a
temperature-dependent resistance, and the defining property of a resistor is
that it be ohmic.