It would be very naive if Madrid did that because in the end we also cannot assume that Bayern is an ordinary club, especially with the release of the Bundesliga trophy at this time, so the goal is the Champions League because it is the only way for Bayern to save their face so surely they will try as hard as possible to get good results so it is wrong if Madrid considers Bayern underestimated in the semi-finals this time.
But for now with Madrid winning against Manchester City some time ago, their probability has increased and they are the favorites in this competition.
I personally still feel that until now Madrid is the favorite for the Champions League and indeed I feel that now is the place for Madrid to get their 15th trophy.
There's no reason for Madrid to underestimate Bayern
They have always been one of the most dominant team in the UCL in recent years so caution would be employed while playing them no matter their form.
Madrid can be considered a favored child in UCL
No matter their form
They always are the likeliest to win the trophy and who ever manages to win them has already won the UCL.
Qualifying for the in the Semi finals alone is enough reason for any team in the UCL to not underestimate their opponent. That was the mistake Arsenal did, after scoring 2 goals in the first leg against Bayern they dashed to goals to Bayern back with a silly mistake. Real Madrid need to be careful here. Harry kane and his squad are very tricky, i still cam remember that goal they scored against Arsenal in the second leg. The header was fast and the goal came from no where straight inside the goal post.
On the other hand, Bayern is the club here with the lesser chances of wining against Real Madrid, playing defensively should be putting more into concentration. This is because Madrid has good and dangerous striker. They are the beast when it comes to UCL competitions.