Because Grass-type Pokemon are the natural enemy of Ground-type Pokemon, it wouldn't seem to be a good fit. However, Marowak in the Alola region apparently adapted by being able to light flames on both ends of the bones that they carry. I'm not sure exactly how they became Ghost types as well, but this is probably a tribute to the original Marowak seen in the Red and Blue Lavender Town Pokemon Tower that could only be seen properly with the use of the Silph Scope. They are also 25% lighter than their original forms, probably due to become ghostly?
Obviously, this is a drastic retyping. The last Fire/Ghost Pokemon that I can think of off the top of my head is Chandelure from Black and White. That turned out to be an extremely powerful Pokémon and actually proved itself in competitive play. While Marowak had always been a cool Pokémon in their design, he wasn't really one to build your team around. Now they have great typing and potentially another unique move or two as he did with Bonemerang and Bone Club in their normal form. I'm really excited to see how trainers utilize this Alolan Form Marowak in Pokemon Sun and Moon. Myself, I'm definitely looking forward to training one.