I don't write horror, but I think it depends on what kind of story you want to write. Do you need your character to be "scary" for the plot? Because I think either way will work. Then again, I don't write horror.
In "It" the MC wasn't scary. The main characters there were a bunch of kids. The scary character was the monster they were fighting. In "Poltergeist", another classic horror tale, the MC is also not scary. In all Lovecraft stories (more classics) the MC is not scary.
Your turn to name horror stories with scary MC, and then just compare with the above, and decide.
Me thinks you're thinking about movie posters rather than the actual story. Freddie Krueger, Jason, Swamp Thing etc were all the title monster which sold the story but not always the MC.