No, it is a myth. Being left-handed or right-handed doesn't make you more or less intelligent than other people.
People who are left-handed have the same amount of brain matter as right-handed people. They also have the same amount of gray matter. Gray matter is the part of the brain that handles movement, speech, and thinking.
Some people think that lefties are more intelligent because they're usually good at certain things like music or art or sports. But all people are different, and this is just a stereotype that doesn't apply to everyone.