Software is what sells hardware, portable or not.
Mario sells hardware. The problem isn't mario, what would be a problem is the other games.
Look at the DS vs 3DS. Top 10 games.
Mario - ~30% attachment rate
Nintendogs - ~25%
Mario - 23%
Brain Age - 21%
Pokemon - 19%
Brain Age 2 - 16%
Pokemon - 14%
Animal Crossing - 12%
Pokemon - 11%
Mario - 10%
Now lets look at 3DS.
Mario - 33%
Mario - 30%
Zelda - 15%
Nintendogs - 13%
Monster Hunter - 7%
Pokemon - 4%
Pilotwings - 4%
Super Street fighter - 4%
Star Fox - 3.5%
LEGO Star Wars - 3%
First let me say, holy shit zelda made it in Top 10, I wonder how long that'l last. Second, notice a trend? Mario makes Nintendo their money and the attachment rate of the 3DS (first party and 3rd party games) is attrocious atm.
It's the hardware that is the problem. The 3DS itself cost Nintendo alot of money. Money that is not getting offset enough by the software. Bad economy, high yen, and the price cut. There's your answer.