I don't really think you can apply stereotypical terms like "dark" to MM, it's just too unique, nor can you really compare it to other Zelda games for the same reason. There are a whole mixture of controversial feelings, with the suffering and pain of the people as well as the happiness they are brought by Link, not to mention Skull Kid and his problems. However, I'd actually say TP is "lighter" for one memory that comes from it. At the very end, during the credits, you see all of the people of Castle Town celebrating, and I remember thinking, "Why the heck are they celebrating??" After you clear the Twilight from the land, nobody is afraid anymore, everything goes back to being normal, people go on with their daily lives, and Link's actions no longer have an effect on the people, more or less. They didn't react to the castle being cut off like it is, Link's quest belongs to himself and his friends alone. So why do they celebrate? Past the first three dungeons, nobody was really in any danger except Link, which means that the tone of the story drastically changed, and all of the sudden the game isn't dark anymore, it's just . . . standard, for lack of a better word.