Going down a German history rabbit hole recently and the Hohenstaufens are definitely my favorite of the German dynasties so far but reading about good old Fred the Red I can't help but feel that his legend outweighs his accomplishments, for a ruler that lost a lot, and whose dynasty died out only 3 generations after him he sure has a lot of presence in the German historical narrative, being one of Germany's greatest mythical heroes, having more statues or portraits than any other medieval German ruler, and was even chosen as the namesake of the largest invasion in history by the Nazis, presumably inspired from their own sick and twisted imagining of German history.
I agree. All in all he wasn't terribly successful considering the affairs in Italy and with the Papacy.
However, in Germany his reign was considerably more peaceful compared to what came before and after him.
I think a couple of things came together to forge his legend:
supposedly he was a very charismatic personality being liked and respected by most of his vassals.
He ruled for a long time, considering he sat on the throne in his 30s which also speaks to his ambitions... he wasn't ever a boy king, raised to rule. It was unlikely for him to become king at all.
Then there is his imperial ambitions that make him appear as the likes of Charlemagne and Otto the Great: reinstituting imperial law in Italy, trying to limit the Papal power against the imperial one again which was in dispute for over 100 years at this point.
And lastly and maybe more importantly his long and relatively peaceful reign coincided with the height of medieval chivalric culture which he was all about.
The imperial diet of Mainz he held was the Woodstock of his time. It was a massive event and everybody of rank was there.
Later generations would probably become nostalgic thinking about his reign and transform it into a time of true greatness for the empire.