Is it correct to say that a bulb in a circuit is a resistor?

Yes it is. The filament in a standard incandescent bulb is a type of resistor. An incandescent light bulb contains tungsten which reduces electricity and converts electricity to heat and light. All incandescent bulbs are resistors, but only a fraction of resistors are bulbs.

If you want to see if a bulb is a resistor, try adding another bulb in series without changing the voltage. Both bulbs will be very dim. Another way to test this is to get a multimeter and set the meter to the resistance setting. If you get any value other than zero, then it is functioning as a resistor.