I highly doubt it. Just because a lot of people have it doesn't mean it is the most common. A lot of people have the World Eater blade but it isn't the most common, right? And besides, there is one way it could be the most common. It is the ONLY legendary dagger, last I checked. So it is the most common legendary dagger, but not the most common dagger.

I agree, Congrats on finding a legendary weapon. It may be more common to see people have them, but it is not a common weapon.