If your state has emissions testing for vehicles and your catalytic converter is clogged and needs replaced then it would be worth it to replace the catalytic converter.
A bad catalytic converter can cause an increase in fuel consumption as well as engine performance issues such as rough idling, loss of power, poor acceleration etc.
If the car is not worth much and you plan to scrap or trade in the car soon then no it would not be worth the cost to replace the catalytic converter.
If your state has no emission testing then you could just have the catalytic converter removed from the exhaust system and there's really no need to have a catalytic converter if your state doesn't require them.
Replacing a catalytic converter can cost from $1,000.00 to $3,000.00 for parts and labor so only replace the catalytic converter if needed.