The original Luigi's Mansion wasn't that much darker than Dark Moon was, in either atmosphere or character designs. People often say the ghosts look creepier or something, but come on, most of the ghosts were rather cartoony creatures created by Vincent Van Gore from magical paintings, and looked about as goofy as you'd expect:
Meanwhile the portrait ghosts themselves, while awesome, were really not all that scary either. I guess Bogmire may be the creepiest thing in that game, and that was more of a reflection of the mansion's hatred than a ghost.
Really, the only thing Dark Moon lacked were the portrait ghosts, and Luigi's Mansion 3 basically includes those now.