Generally speaking, don't ever unplug the MAF with the engine running. Unplug it with the engine off, then start the engine.
Your results indicate that the engine is getting a different (more, probably) amount of air than the MAF is telling it. With it unplugged, the PCM goes to some (mostly safe) default assumptions about how much air it is getting.
So, yes it could be that the MAF is bad. However, it is more likely that you have an air leak somewhere from a cracked tube or disconnected hose. I say this since MAF failures are fairly rare and you said that you just replaced valve cover gaskets. That job involves disconnecting and moving some brittle PCV tubes and hoses. If you have a propane torch, turn the gas on, but don't light it. Now with the engine running, pass it all along the top of the engine where those tubes and hoses are. If you get a change in engine speed/sound, then you have found a leak.
