As a German: It depends on the context. If you use it as a noun, referring to an individual act, it is used to refer to a mistake. But even then, not a silly o

As a German: It depends on the context. If you use it as a noun, referring to an individual act, it is used to refer to a mistake. But even then, not a silly one. It refers to a lapse in judgment, to a lack of proper reasoning. If it doesn't refer to an individual act, but to a trait, it's meant as an incapability to reason properly, often painfully so. It's used as a derogative, not to mark a silly mistake. The most famous quote about Dummheit makes fairly clear it's not just meant as silliness. Schiller's "Mit der Dummheit kämpfen Götter selbst vergebens" (Even the gods struggle in vain against stupidity) clearly marks it as something both detrimental and incorrigible. That's really the core distinction - Dummheit as referring to an individual act vs. as a trait. This goes for both the adjective ("dumm") and the trait. It is an implicit acknowledgment of the fact that individual mistakes happen and are excusable, but constant failure is a negative mark. Bonhoeffer very clearly refers to the latter, but moves it from an individual failing to a societal one.