What they said.
The horror setting of Cthulhu and Ravenloft are quite different in their atmosphere. While Cthulhu's horror lies in the senselessnesss of a world that is just a playground of dark gods, Ravenloft has a clear distinction between evil vs. good - black and white.
The Dark Powers seem to fit the Cthulu mythos. The rest of the setting fits more with Lovecraft's other works. They even scte several of his short stories as examples of ravenloft type horror like "rats in the walls."
Er, no. The Dark Powers, for whatever reason (read: up to the DM), are interested in morality--the entirety of Ravenloft as a setting revolves around morality, the seduction of evil, the horror of man's inhumanity to man, the nature of sin, etc. The Cthulhu Mythos' horror comes from the idea that human morality is meaningless, that the universe is amoral and the beings that run the universe are either hostile or indifferent to humanity. This is almost the complete opposite of what Ravenloft is about. The only part of the setting that fits the Cthulhu Mythos type of horror is Bluetspur, and that's an exception that proves the rule. Some of Lovecraft's works are good inspiration for Ravenloft, but it's usually non-Mythos material such as "The Rats in the Walls" and "Pickman's Model"--works that don't include the Cthulhu Mythos "deities."
I've always thought that the Dark Powers were "hostile or indifferent to humanity." The Dark Powers are basically alien in their purpose to the people of ravenloft. They do what they do for some unknown purpose. Whether it is to test morality or simply to entertain themselves is not answered. I would say their not interested in morality so much as depravity. The only focus on the one side of good/evil: evil. Good isn't really rewarded, it's just twisted into new evil. Essentially, everyone in Ravenloft is doomed to the whims of dark unknowable powers. That's why I draw the comparsion between them and the Cthulu mythos. In the gazetteers, players' guide, and novels, that's how they're always portrayed.
Incorrect, the Dark Powers are never portrayed except for in one novel,
Lord of the Necropolis. This novel was quickly rendered non-canon by the staff at TSR because of this. The is no true definition of what the Dark Powers are, but to say they are "hostile and indifferent to humanity" is a stretch. If they were truly like the Cthulhu Mythos, they would not bother cursing the darklords, tempting them with their innermost desires only to dash them whenever it was within reach. To do such a thing requires a level of understanding of human nature that the shuddering alien monstrosities of the Mythos cannot and will not ever have (save, perhaps, Nyarlathotep).
To say that good is never rewarded and that everyone is doomed is also false and not supported by the works. Rudolph van Richten found redemption for his curse, the Carnival rescues lots of poor lost souls that would otherwise be destroyed, and in a PnP game the player characters can be rewarded with escape from the demiplane (as in the end of
Roots of Evil), and so on. To sure, there is more darkness than light in Ravenloft, but those lights burn brighter and longer than any of the inexorably insane investigators of a Call of Cthulhu game.