Myrrh and def smoke. Is it a good idea?

A +Def Myrhh has a defense of 42 with her weapon right? I guess it'd be good for arena because of all of the armors though

I can see that being a good idea... With good positioning and the right buffs, you can bait out anyone who is not L!Marth.

It's not bad, but you won't be able to active the skill against the opponent that you fight. I'm thinking that Def Ploy will be better for lowering the opponent's Def since you don't have to engage in the first round of combat to trigger the Smoke debuffs. You can even have a combination of both: Def Ploy to hit the initial target, then Def Smoke to hit everyone else nearby that didn't get hit by Def Ploy.

While Myrrh could even run both skills in her build, I suggest that she doesn't run Def Ploy unless she has Fury equipped (which seems not to be the case for your Myrrh) and/or pull a +Res Myrrh; otherwise, give it to someone else and just have her run Def Smoke. (You also get to keep Iote's Shield instead of giving it up for Def Ploy.) If you are still worried about not being able to activate Great Flame's effect, you could also switch Iote's Shield for Close Def / Distant Def.

Who are you teaming her up with? If dragons, then the native Hone Dragons is going to be better most of the time and the dragons will by default target Res, unless ranged foes of course.

If you put her in a more mixed team then her allies might benefit from the smoke effect too so it would not be just to keep Great Flame online. An ally could also run a Def Ploy or Chill Def to further assist.