The quality of player is what many do think can get a team trophy but this proved to everyone big time that there is more to a team than just having set of good player alone, Inter Milan had their confidence that way because they probably were not expecting PSG to be a problem to them , if you had followed up with how PSG made it to the finals you will agree they struggled so hard at some point, even at the league stage they finished below top 10, so maybe Inter least expected much challenge from them.
You have said it all. Inter underestimated PSG due to a lot of reasons. Firstly, they haven't win the trophy before and don't have the finals experience. Secondly, they struggled till they got to the finals and lastly, Inter defeated Barcelona who everyone sees as the best last season. I believe that Inter didn't even prepared properly for that finals. Assuming, they prepared for the finals the way they prepared when they played Barcelona, PSG wouldn't have humiliated them that way.
It is certainly true that Inter could be underestimated by the French club for many reasons, as they were defeated by them in the Champions League final despite having prepared properly. However in the upcoming season, it seems that the Italian club may have lost the ability to reach that level, as they previously played with great skill against the Spanish champions, As a result, many people continued to speculate thoughtfully that they could not perform in the final as neatly as they had when participating in the match against Barcelona.