It means stupid, in American English they say "dumb" more often, in British English "stupid" is more common, although dumb literally means not being able to speak. Dumb as a brick is just a longer way of saying dumb, in the UK they might say "thick as 2 short planks" and there are other ways people express this depending on region and dialect. As for stable genius, well that's just Trump, not known to be modest. I don't think many people would say this of themselves, it sounds arrogant.

1. It means unintelligent. "Dumb as a ______" is a common idiom. The blank is usually an inanimate object, so "Dumb as a brick" or "dumb as a block of wood" or "dumb as a bucket of hair." 2. It is not a thing that people commonly say in this culture, either. Why he's saying it is something that only he could answer.

Hi there. I laughed quite hard at the question if I may say so, because of the people involved in the question. Firstly, when "as x as y" is used, it is known as a simile. E.g: As cute as a kitten This is to make the description of something more relatable to someone by giving an example of objects and subjects it could be used to describe. Hence when Bannon says "(as) dumb as a brick", he is simply inferring that both Ivanka Trump and a brick has the same level of intelligence. Why he thinks a brick could be dumb is anyone's question. ¯\_(ツ)_/¯ As to the second question, I think that most people will not call themselves a "genius" unless they are really arrogant... For example, Trump. I doubt most westerners are as arrogant as he is haha.