Talking about luck I don't know if PSG can be lucky enough to beat Inter Milan in the UEFA Champions League final, but if it's 3 or more goals, it's likely to happen because both teams are two scoring sides, and they can possibly play BTTS, which can lead to over 2.5 goals. Inter Milan is a club that always gives their opponents' point man a close marking so that he will not be a threat to them, so since Dembele is a point man for PSG, I am not sure if he will have the chance to score a goal tomorrow.
Why you doubt PSG to win the final match? PSG is strong enough to beat Inter Milan, they are no longer the same as PSG team in the past.
Although these teams scored many goals in the previous rounds, there is no guarantee that they will score many goals in the final match as well. I assume they will be more careful, they probably will be a bit defensive in first half of the match. We also must be aware that both teams have strong defense, so it won't be easy for the attackers to score goals. I think it is possible that we only have 1-2 goals in this final.
Dembele performance was fluctuating a lot in Barcelona then he moved to Paris Saint-Germain and started to improve and gets his form back.
In Barcelona, he didn't get enough time to play. He wasn't play a role as the main goal scorer. He was only ordered to serve the striker (Lewandowski). So, there was no enough space to improve his ability in Barcelona.