Do ghost type Pokémon appear only at night?
Solution 1:
No, it's not. For starters, here's a Gastly around 4pm in the afternoon.
I've also been playing the game until 2 am every night since the game was released and haven't seen a single ghost-type Pokémon in the wild at night. It is doubtful that time of day is a factor for them, which means they're probably location dependent, just like other Pokémon.