http://www.snopes.com/supreme-court-justices-stand/ a complete fabrication is 'mostly' false. "What's true" is not even relevant to the claim.
Per their stated rating system [1], 'Mostly False' is defined as "This rating indicates that the primary elements of a claim are demonstrably false."
The slim kernel of truth in it is that the fake letter references a real court case involving the Supreme Court and Gorsuch - which seems like why it was only Mostly.