The word "stupidity" is confusing, as people use it to refer to different things.
1) People can do stupid things, because they are mentally incapacitated, either permanently (e.g. mental retardation) or temporarily (e.g. drunkness, tiredness). There is nothing they can do about it, so it is morally neutral. In case of drunkness, the moral failure could be getting drunk, especially if you know it will result in you doing stupid things.
2) Willful (and often joyful) irrationality. This is the kind that Bonhoeffer points to, I think. I agree that it is a moral failure, but for many people "morality" simply means "what my friends approve of"; so if they get irrational together with their friends and in the same way, they see no moral problem in doing so, and their friends would agree.
3) People can also do stupid things simply because they are misinformed, either by mistake, or by being manipulated. That again is morally neutral. If it was a mistake, and they get feedback that they are wrong, now they have a moral choice: either start thinking and stop doing stupid things, or double down (which now makes it willful irrationality).
Yes, from this perspective, many people are immoral. Note that even a group that seems "homogenously stupid" from outside can still contain all three kinds of members; some members are willfully irrational, others only made the mistake of trusting the former.