as I posted before,
I wouldn't say it's scary in the traditional sense. It's harrowing and emotional throughout. You develop personal involvement with each character throughout the days, and you witness characters go through the stages of denial and grief before accepting (or not) their certain deaths. You help ease the pain of a wounded character, who dies. You then turn his soul into a mask and transform into him with a twisted cutscene of you screaming in pain. You encounter a mother and child who have to accept that they will both die the next day. You meet characters who stay upbeat and positive for the first two days, only to end up cowering in their rooms during the final hours of their life once they realize that yes, they too are going to die and no one is going to save them.
There is a constant feel of dread and the atmosphere is constantly foreboding, dark and twisted throughout. The few elements of happiness are tinged with a surreal and disturbing feel. You help a separated couple get back together and marry in the final moments before the world ends. It's beautiful, yet bittersweet because even though they are now together, they have to comfort each other knowing the world will end in a few hours time, declaring they will not run or separate again and they will "see the morning arrive...together".
The antagonist, Majora, has no motive other than to cause pain and suffering by killing everyone. Even after you save the world, certain characters still have an unhappy ending - such as one who's son is murdered by Majora before entering Termina (and as such, Link's time travel could not fix that).
Hope that all helps you understand it a bit better. Twilight Princess is the Teletubbies compared to Majora's Mask. And Majora's Mask is certainly without a doubt the darkest and most disturbing game Nintendo have ever developed.