@NatsuI mean, I suppose they might have
some effect. Maybe just not the desired effect, as Viola fundamentally has different life values than what most people would consider 'normal and happy'. She likes those negative emotions. She finds anger - even her own - fascinating. Sadness, depression, to cover something like that up is simply turning a blind eye to it. In fact, she'd probably find Melody's powers completely abhorrent and a mockery of human nature.
Happiness to Viola is watching others screw up - even more so if she's the cause of it, or meddling in affairs and just watching the world burn as a result. Or maybe it doesn't and everyone ends up singing along happily. Either way is fine for her - she just wants a giggle out of it.
So yes, I would say it could make Viola less sad or disappointed if one of her pranks or plans didn't pan out, (though that is unlikely as Viola isn't the type to dwell on things. If anything, she'd laugh) and it might have a small effect on her jealous tendencies, but unless the magic could make Viola change her fundamental beliefs and life views it would only be a temporary effect, I think.
The big thing here, is that unless it can cause Viola to suddenly start feeling happy by doing altruistic or traditionally positive actions, then it's not going to really work well as she finds that stuff boring, trite, and against human nature.
