No, it's not normal on a well maintained rifle, I've seen Mauser rifles and "Mauser Patents" rifles...think the British Pattern 14...that are now a hundred years old others from WWII and the extractor is as good as the day it was made. Save for loss of blue.
Pitting is loss of metal caused by failing to remove rust. So under that rust the metal is attacked. That metal, now gone, is gone for good. "You failed to maintain your weapon"...as they say. However it won't affect how it shoots.
This is perhaps too many folk on here "boasting" about how they only clean their rifle once a year or such and so. Mine gets cleaned every outing regardless of if it's fired a round and the exterior surfaces wiped with an oily cloth.
I am sorry if it's "tough love" but it's down to poor maintenance at some point in its life. Either a previous owner or its current owner. Only you know who the culprit is.