Is that really true though? Even with their powers they'd still have to be physically active to move and jump around, unless their bodies are literally just puppeted around by their powers.
If that's the case, their bodies would basically have no muscle mass at all. Without their powers they'd be frail and practically immobile, like someone who had been in a coma for years. Even a couple months in Zero G can cause astronauts to need physical therapy to rebuild muscle mass, so imagine a creature that effectively never leaves it.
But besides that, I imagine they'd also have to deal with heat from exterior conditions. If you put your Gardevoir in a blanket and turn up the heat she'd need to cool down somehow.