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.
I don't agree with you that Inter Underestimated PSG and if you insist, you have to tell me from which point they started underestimating them and what is the proof. What happen was that PSG was more determined to win the match and the showed this from the beginning of the match to the end. Inter Milan were overwhelmed by PSG from the beginning of the match to the end, they were never allowed to play the ball and were loaded with enough goals to convince them to accept defeat. 
Especially when the tournament is as big as the Champions League.When someone says that Inter Milan underestimated PSG in the final, I think they are either unaware of the momentum of the match or they don't know it. PSG took the momentum in their favor from the beginning and they were able to keep the momentum in their favor until the end. Inter Milan didn't have time to find the momentum of the match and they couldn't build any competition in the match. It wasn't really Inter Milan's underestimation, but rather PSG's determination and confidence.