I agree with the person who said Quayle was smarter than his reputation. I always thought most people missed the boat completely on Quayle. The problem wasn't his intelligence -- it was his extreme conservatism.
I believe if he'd become president, he would have been the most conservative president of the post-war era -- up to that time. Whether he's more conservative than GWBush, I don't know.
If he'd become president in early 1992, the question is, is he the Republican nominee for president later that summer? He probably is, if only because there just isn't time for anyone else to mount a serious challenge to him. Pat Buchanan is already in the race, but he just turns off too many people. Some Republicans, in private, might be thinking about a way to deny him the nomination, but I don't know if they succeed.
If he is the nominee, the combination of Quayle's ultra-conservatism, the weak economy, and his perceived lack of intelligence cause him to lose in November of 1992.
The only variable is this: of course, there will be an outpouring of grief for President George H.W. Bush that makes him much more popular in death than he was when he was alive. At the same time, new President Quayle has a honeymoon with the American people. Does this change the political landscape enough to enable Quayle to win in November of 1992?
I say probably not. By the fall of 1992, the honeymoon has worn off, and people see President Quayle as an ultraconservative lightweight leading America at a time when the economy is sluggish. I say he still loses.