It certainly is! Your auto's air filter is the only line of defense in stopping airborne contaminants from getting drawn into your car's air induction system. The air filter stops these particles from getting into important fuel injections areas and damaging those parts or plugging them up. As the air filter gets filled up with contaminants it's filtration ability is maxed out - at which point, contaminants will begine getting past it.
A plugged air filter can limit the necessary air flow needed by your engine to run correctly. When this occurs, your car's engine will run rich and will use excessive amounts of fuel reducing car's miles per gallon in the process.