First of all, in this particular encounter between borussia dortmund vs real Madrid in the uefa champions league final, it very clear that real madrid are the clear favourite here, so the bookmarkers odds between this two teams are not wrong, and I believe that the odds of real madrid will keep on getting smaller as the day of the match getting closer, while the odds of borussia dortmund will keep on increasing, because everyone expected them to be defeated by real madrid, but one good thing about football is that anything can happen, nothing is guaranteed, borussia dortmund can surprise them on day and go home with the trophy if real madrid is complacent on that day or they think that since they are the favourite her, borussia dortmund will be a pushover.
And talking of borussia dortmund not deserving to be in the finals? Yes they actually deserve it, though they aren't that strong, they were mostly lucky in their journey to the finals, because the teams they met on their way wasn't that strong as the team madrid met, but luck is one vital thing that is needed to be successful in this competition, so they actually deserve to be in the finals.
I do understand that we are going to end up with Real getting the win unless some big shock happens and everyone knows this. I am not saying that we are going to have Dortmund at zero chance, lower odds teams managed to win, just a few years ago I have seen 12+ odds win against Real Madrid, a team called sherrif that I have never seen again ended up beating Real Madrid, however at the end of that year Real still managed to win the cup, but still weirder things happened.
So all in all, I would say that just like how Atalanta managed to win Leverkusen who went 51 games unbeaten (52 total) in a row, we could see Dortmund could win as well. But are we really willing to put our money into that? I am not, I rather put my money on Madrid to win.