I've met this phrase in The Big Bang Theory TV-series, it was a phrase of Sheldon's mother after she forced him to go to apologize to the boss to get his job back:
Leonard, the Lord never gives us more than we can handle. Thankfully, he blessed me with two other children who are dumb as soup.
From the context, it's clearly that it means very stupid. But why? Word soup doesn't associate with any kind of stupidity to me. Also, I haven't found any additional meaning or idioms with this word fitted the situation. Is here some play on word involved? I have found other discussions in the internet about this subject but with no real answer.
btw: I can't force myself to hear soup in that moment. I always hear something like soum, though subtitles say that it is definitely soup word.