I'm gonna say what I normally say in every ditto matchup
Ditto without transforming has skill on par with the Machamp line, who regularly train in and have mastered every martial art.
It's also capable of copying the likes of Decidueye and Marowak. Decidueye is a great archer and Marowak can throw bones like boomerangs which doesn't even make sense, so it's likely that Ditto can match her marksmanship
Ditto can also copy the likes of Sirfetch'd, Gallade, Ceruledge, and other sword-based pokemon so it should far outclass her in swordplay
And due to the aforementioned martial arts mastery, Ditto folds her in half if she gets close in any capacity
Ditto also doesn't copy her clumsiness so I think that will be her downfall in this matchup against herself
voting Ditto for now, idk much about Marcy so I might be wrong about a few things