It seems to me that there should be no such thing as underestimating an opponent at the Champions League playoff stage ,any team that was able to pass the group stage should by default be an opponent that is worthy of respect. Bayern simply were not ready for Lazio to play so well in defense, I think this is the main reason. But Bayern have the second leg where they will have a chance to turn things around, the difficulty for them may be, that their defense is not the best, and if Lazio can score again and try to play just as well in defense then there could be a sensation.
Yes, it is absolutely true that no team will underestimate its opponent in the round of 16. In fact, not only in the round of 16, but since the group stage, everyone has competed without underestimating their opponents. Because everyone realizes that the teams that managed to get tickets to the UCL are the best teams. And in the round of 16 things got even more difficult. So there is no reason to underestimate your opponent. And yes, Lazio managed to win because they were also ready for the match. And Bayern lost because they lost in creating a strategy and in executing the opportunities they had. Or this is really about which team is better prepared and more mature in making preparations.