If ditto uses transform on an Arceus, it will look like the water Arceus, but if Ditto wasn't holding the plate, Jugement would be Normal, however, Ditto is holding a Fire Plate, which makes Jugement a fire type move, but Ditto also copies the type as well.
1) Jugement is a Fire Type move
2) Transformed Ditto will still be the Water Type Arceus.
(Sorry I edited it, I just found out yesterday about ^ and so I corrected it. ^.^)