It means he would win by a great deal. He would win by a large amount. If there were 100 people voting for biggest mama's boy, he'd probably win all 100 votes.

Many English expressions are figurative and based on physical things. Always ask yourself what the literal meaning of a word or phrase is and it will help you understand it. In this case a literal, physical landslide is "a large mass of rocks and earth that suddenly and quickly moves down the side of a mountain or hill. Examples: The earthquake triggered a landslide. They were buried under the landslide. (From the online Learner's Dictionary) So, to "win by a landslide" is to win by such a large number of votes that the win is as huge as a landslide.

Usually students elect a Class President . He´s saying if they elected a Class Clown , then he would win that election with a mountain of votes,that is,a lot of votes...a landslide.