Joan's attack debuff is -20%/A-frame. So it would stack with Sol's -20%/C-frame, but not Belobog's -15%/A-frame, or Urania's A-frame atk debuff. I'm not going to assume that you have Sol. The -40% damage cut would probably be more desirable later, if enemy design changes up? Which is really the main issue with the defender type in the state we're at; we've yet to have a situation that really calls for a damage cut skill.
As it is right now, Joan's biggest appeal is the atk debuff, which plays up better when you have B or C-frame debuffs instead of A-frame elsewhere (Cybele's Mistletoe Drain being an example of B-frame). Of course, that means you can just pick it up to be used as an extra skill on a different soul sooner or later.