I still think Real Madrid is quite serious in building their team this season because their less than optimal performance lately does not mean that they should judge them as not serious in playing in every match. But it is more about the performance of their players who are a bit chaotic because there are old players who have retired and also there are new players who may not be used to the tactics in Real Madrid. So I will not judge them for seriousness or not because in reality they are really serious in every match even though the results may look different for this season.
More precisely because some of the players they have are injured so that Real Madrid's strength is much less consistent. They have lost twice in the Champions League so far and the defeats were against teams that were not too strong so there are many things that Ancelotti needs to fix and that is not talking about the draws they have gotten. Real Madrid remains the favorite in the Champions League because they do have a very good mentality in the league.
But if their strength is not much better and they do not play more solidly then I doubt they will be able to maintain consistency and it will be much more difficult to defend the title because currently there are many other teams that may have changed with much stronger and more solid strength.
The injuries of players that continue to befall the Madrid squad at the moment, greatly affect their performance and game, the injuries of some key players have made them lose their consistency and strength, what you say I think is quite true, the 2 defeats they got in the UCL this season have shown their performance has declined drastically, let alone the defeat they got was from a team weaker than them, So Ancelotti should start trying to find the solution from weaknesses of his current team, because if not then it seems that we have to exclude them this season as the favorite teams in the UCL.