The checkmark meaning “correct” comes from Latin, it developed from V for veritas - truth. Japanese (traditionally) does not have a sound corresponding to V, that’s why B is commonly substituted, a sound that is phonetically close to V. “Idiot” of course means 馬鹿 baka so a V(/B) next to something was misunderstood as calling it wrong and idiotic. The misunderstanding stuck so now a checkmark is used for mistakes instead of correct things.