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.
That was Inter's weakness last season underestimating their opponents especially after defeating Barcelona in the semifinals. Inter also considered PSG to lack the mentality and lack experience in finals. This was Inter's fatal mistake last season, leading to their humiliation by PSG in the final.
Even though PSG hadn't won the UCL they were coached by a coach with experience winning the UCL. This should be a lesson for every team in the UCL this season never underestimate your opponent because in the UCL competition all teams are of equal quality and represent the best teams in their respective leagues.