I love the way Pokken made the design. It comes very close to the way I think gardevoir would look like in the real world.
The only thing I would change are the ears and the 'dress'.
The ears (those three spikes on the side of her head) should be just a tad bit longer in my opinion.
The 'dress' is shown to be a part of her body. Even if it's not the case, that's how it looks like. In my opinion, the dress is an actual piece of clothing they wear over their bodies, so I would probably change the design in a way that would show this clearly.
Apart of that, I think it's a pretty cool and realistic design. I'm glad they didn't interpret the hair as helmet and actually make it jiggle and all that, showing that it's actual hair.
Honestly, with the way DP handled the Pokémon designs, I'm sure Gardevoir's design would've been pretty good. My only fear is, again, them showing the hair and dress as part of their actual bodies.
Overall, I think it'd look good, but I cannot say that for certain.
Thank you for the lovely question.