IIRC Dark types aren't
evil so much as being more 'animalistic' and thus instinctual\violent compared to most other types of Pokemon,
on average.
This gives them a
reputation for 'evil' because they tend to come across and vicious and cruel, sort of the same way that cats get a bad rap for 'playing' with prey, despite cats not possessing the necessary thinkspace to understand morality and having perfectly good practical reasons for tiring some types of prey out before going for a kill.
Ghost types on the other hand are ghosts, they run the whole gamut of ghost stories from evil monsters that actively hunt souls to eat (and\or drag off to hell) while causing chaos and destruction for their own personal amusement, to a haunted teapot that makes really good tea but only if it likes you, to the soul of an ancient warrior possessing his ancestral sword that mentors the worthy and drains the life from the unworthy.
(Steel\Ghost is a ridiculous typing btw, Aegislash is such bullshit.)
No Pokemon type is inherently
evil, types like Dark and Ghost can
seem evil on a cursory inspection, but that is just a misunderstanding of what traits those types actually possess and what traits vary from individual to individual.
There's a step in the process where you literally add love to the stew. Maybe that's the part that's nutritious for them.
Most likely; Ghost types are the type that most often gets an explicit line about what they eat, and almost all the ones that do get that line eat some kind of mental or spiritual aspect of other beings; souls, life-force or emotions usually.
So probably when you feed a Ghost pokefood, while their 'physical' body (in so far as some Ghosts have one) might eat the physical food, the actual nourishment or benefits that the Ghost gains from doing so would come from their consumption of the
soul of the food, which would presumably contain a degree of the emotional investment put into creating said food. Hence the adding love to the stew being a necessary step in cooking for Pokemon; some Pokemon
literally eat that love.