> This is mostly used incompetently, but occasionally used maliciously.
It can also just be a good heuristic. If a person or group/org with a reputation for dishonesty tells you an "idea", it's reasonable to throw shade.
> I think I see this most commonly in politics, where if <obviously bad> person supports an idea, then that idea must be also bad.
The conclusion is not always this direct, but it is completely reasonable to question ~why~ the obviously bad person holds that idea. What do they gain from pushing the agenda? Some people are so intellectually dishonest that everything they say is suspect.