Both have bad '70s haircuts.
Both were abandoned as children in the desert.
Both have trouble after finding a meteorite.
Christopher Walken audtioned for the part of Han Solo, who helps Luke. Christopher Walken plays the janitor that helps Joe Dirt.
Luke is thrown into a deep dark pit by Jabba the Hutt. Joe Dirt is thrown into a deep dark pit by Buffalo Bob.
Both meet an old wiseman with a speech impediment in swampy places (Dagobah, Louisiana).
And of course, the number one similarity: both are sexually attracted to their sister.