Mature as in graphic violence & sexual content? No, they don't need too. They let the 3rd parties do this for them. Would Nintendo do it? Very doubtful, Nintendo of America try's very hard to keep a clean family image. Though as a few have noted, they might let one of their first party or second party Western developers make a game like that for them.
Mature as in story? Sure, I would like to see Nintendo do more in this area. They have a few games that have mature stories like Metroid, Fire Emblem, Advanced Wars, Mario & Luigi RPG games, Paper Mario, and about 1/3 of the Zelda games. Though I'm not sure they will create too many new IP in this area any time soon, since their studios are having a hard time cranking out the main Nintendo franchises fast enough to keep consumers happy.
Marture as in adult appeal? Not really. I think they already have this field covered. From Wii Fit, Brain Age, Wii Sports, New Super Mario Bros, to about half of their library. Nintendo generally makes games that appeal to adults. Though many of their games have a cartoon sheen on them and are rated E for Everyone, adults like me still buy and play them.
