~を笑う (laugh at)

The verb 笑う ('to laugh') takes を to mark what is being laughed at — 'laugh at X' in English. 人を笑う ('laugh at people'), 失敗を笑う ('laugh at the failure'), 自分を笑う ('laugh at oneself'). No preposition is needed because 笑う in this transitive sense directly takes its target.