So, Eddard and Robb Stark inspire alot of loyalty that lasts well after their deaths. Jon Arryn seemed to inspire the same. My question is, was loyalty of this level a common thing in the Middle Age?

It could be. I wrote a piece a while back about how common betrayal was during the Wars of the Roses (by way of explaining why Robb keeping faith with the Freys is a dumb idea), but it’s also true that there were people who fought for either the Lancastrians or the Yorkists for decades, and stayed loyal throughout incredible turns of fortune that saw monarchs go from being all-powerful to exiles in a few months.