They made it to the champions league doesn't mean that it's guaranteed they would have a good experience there and besides no team would take them for granted anymore as they are fully aware of their capability and know exactly they are can do. The champions league this season is no joke and another team everyone would be looking to stop would be Madrid as it would be a complete joke to allow them win again in the champions league.
Teams that are the rivals of Real Madrid should have thought about that before they let them sign Mbappe, since now it is entirely possible the Real Madrid enjoys a period of dominance like the one they enjoyed from 2014 to 2018 in which they won 4 out 5 UCL trophies, and yet they did not do it, so I have sincere doubts about the possibility of any team being able to stop them, especially if Real Madrid is at 100% and its players do not suffer any significant injuries.