Civic poor fuel economy stalling black smoke

Those symptoms definitely point to a potential MAF sensor issue. Have you tried cleaning the MAF sensor itself? Sometimes a bit of dirt or oil buildup can cause it to malfunction. What kind of smoke are you seeing when accelerating - black, white, or blue?

I haven't tried cleaning it yet, that's a good idea! Is there a special cleaner I should use, or can I just wipe it off? As for the smoke, it's more on the black side, I would say.

Use a MAF sensor specific cleaner; they sell it at most auto parts stores. Regular cleaners can damage the sensor. Black smoke usually means you're running rich, which a faulty MAF sensor could definitely cause. Given the stalling and poor fuel economy, plus the black smoke, replacing the sensor is likely the best course of action. It might be time to take it to a workshop and get it checked out.

Thanks a lot for confirming my suspicions! I cleaned the MAF sensor, but the issue persisted, so I took it to the workshop as suggested. Turns out the MAF sensor was indeed defective and it cost me 365€ to replace it. The car is running smoothly now, no more stalling and the fuel economy is back to normal!