Is Myrrh (-atk/+def) good?

A -Atk/+Def Myrrh is definitely still salvageable, considering her breath has a total of 19 might. She will have about 49 Atk which is still great! Just stick a +Atk sacred seal on her.

It isn't bad. Even without her weapon's +3 she still has 30 base attack which is workable, particularly with a legendary weapon. With Great Flame's +3 she has 49 ATK before anything else is taken into account. Plus she targets the weaker defensive stat(RES). Combine that with Dragonstones hitting the weaker defensive stat when the foe is 2 range and 30 base attack is plenty even for a Distant Counter build. Honestly I would gladly trade my Neutral Myrrh for yours.

With that said it does pretty much lock you out of Heavy Blade builds. So if you were hoping to try that, well sorry.