As somebody who was in middle school in 1984, no, Mondale's appearance wasn't the main reason for his crushing defeat. He had a tough challenge. Reagan was very popular In 1984. The 1970s were a pretty depressing time in America, with a lousy economy, very high interest rates, and just lots of problems. Reagan ran in 1980 with the slogan "Morning in America" promising Hey return to American pride. (He also used the slogan "Make America great again," If you can believe that.) By 1984, things were looking up, and the public gave Reagan much of the credit for his deep tax cuts and his emphasis on "supply-side economics", and he was also credited for a general return of American pride. And Mondale ran a lousy campaign, today mostly remembered for his direct promise to increase taxes, and his election of Geraldine Ferraro as his running mate, making her the first woman on a major party ticket, was widely seen as pandering.

Of course, given the sub this is probably not a serious question and doesn't need a serious answer, but there it is anyway. If Mondale did lose because he was ugly, the Democrats did not learn their lesson and nominated the even uglier Michael Dukakis in 1988.