Yeah, Jericho beating Rock and Austin in the same night to become Undisputed champ was legitimately shocking. You only have to listen to the crowd reaction to see that. Up until that point Jericho was not really seen as a bonafide main event talent, and I think most people were expecting either Austin or Rock to go into Mania as champ. In a kayfabe sense, Jericho was looked at as having pretty much no chance, but for the majority of fans, that was the truth of the matter as well. There were some who probably thought he would get through his first match just in the name of shock value, but I doubt there were very many people who were thinking he was going to actually win the belt, especially when it was revealed that he'd be facing Austin in the decider.
Obviously, there were likely a few who predicted Jericho would win the strap, but I'm willing to bet it was a vast minority of fans who honestly believed he would do it. To compare, it's kind of like when Jinder beat Orton. Were there people who said Jinder would win? Sure. Did that make his win any less shocking? Not really.