Tora, had you actually cared to read what Mill had to say about Conservatives and intelligence in my earlier thread, you might just have spotted the following:
"I never meant to say that the Conservatives are generally stupid. I meant to say that stupid people are generally conservative."
I assume you also failed to notice his use of the distinction between Conservative with an upper-case 'C' and conservative with a lower-case 'c'.
In other words, one doesn't - in your words - "have to be stupid to be a conservative"; it's just that, with the bulk of truly stupid people tending towards the 'right', right-wing parties should generally do well in elections.
If Thatcher was so bright, it's odd that she never seemed to grasp why she was so widely detested.