Yes, its a cast mark, or "Flash". You can see flash on most all cast and molded items. Its generally where the two halves of the mold break apart. If you look on the frame of any polymer gun you can generally see where the molds come apart. You can even see it on a lot of AR15 receivers as well.
Its just a normal part of the manufacturing process unless it gets machined down. The edge on that extractor is non essential to any type of function so therefore machining it would have been merely cosmetic. Would it look nicer, yes. Would it add to the cost of the gun, possibly.