Personally, I don't think I've ever thought of anyone as stupid (except for myself on occasion! ;) But any thought such as that towards myself or anyone else would be fleeting.)
I don't know if the word has changed in meaning over the years, but I find someone saying of someone else, he's a stupid fellow, is a rather dismissive thing to say. And I am not sure why "not more than common sense" should be the equivalent of saying someone is "stupid". But I can easier think Edmund said it to himself in frustration with the man.