I am not going to suppose that I know anything about Germany, but here in the USA, the first question the magistrate is going to ask me is, "but just let me be clear, this is a route that you drive often?"
We don't need to be rocket scientists to see where that line of questioning goes.
The onus is on the municipality/government to keep signage readable, but a magistrate that hears you say, "I was going 100kph because the 60kph speed limit sign was unreadable due to wind-driven snow accumulating on it" will dismiss your argument for several reasons: First, you DID SEE the obstructed sign, because you noted that it was a speed limit sign that was covered with snow...so you KNEW that there was a speed limit on the route even though you couldn't make out the numbers. Second, you were driving recklessly for the conditions, irregardless of what the sign read. Third, you KNOW THE ROUTE.