With both NSMB U and Super Mario 3D World failing to move systems, it would really make no sense to assume that MORE Mario will cure what ails the Wii U. But Nintendo has no idea what they're doing anymore so I can see them thinking this will matter.
This isn't news. It's like saying the Activision is making a new Call of Duty or EA is making a new Madden. Yes, I now compare Mario to those soulless cash cow series but he become annualized so isn't the comparison fair?
It seems like Nintendo has become so conservative that they're using the logic that since Mario is their most popular series that they should focus almost entirely on him. That's some really flawed logic but it is the sort of thing corporate suits will think up. They're noticing that Mario can sell 10 million while another Nintendo franchise may only reach 2 million. So if instead of one of each if they go with two Marios they can make 20 million instead of only 12. The fallacy there is that the people that will buy a Nintendo system aren't only interested in Mario. Mario might not even be their favourite Nintendo series in the first place. What Mario is is the common denominator. He's the Nintendo franchise that almost every Nintendo customer likes. Now Zelda or Pokémon or Wii Sports or even something more obscure like Pikmin or Fire Emblem is their favourite and ends up being their killer app but they all like Mario and will buy Mario games if they own a system his games appear on. Nintendo is looking at that large group of Mario players and thinking that Mario is the draw for all of them, but the group for which Mario is specifically the draw is much smaller. This is why Mario made minimal impact on the Gamecube, was a huge success on the Wii, and is again making a minimal impact on the Wii U. The Wii had a massively popular killer app in a new IP in Wii Sports. It had Zelda and Metroid, two franchises with very devoted fans, being released prior to Mario. It had more than just Mario to attract an audience and by the time huge Mario hits like Mario Kart Wii and NSMB Wii came out the userbase was already established.
Mario is a draw but he isn't the reason the Wii sold like hot cakes. If any one game could claim to be the Wii system seller it's Wii Sports but I think for any system it's a team effort of many games that make the purchase worthwhile to the individual consumer. That's why variety is so important because you never know what specific combination is going to sway each individual. That's why third party support matters. That's why you aim to cover multiple genres and multiple demographics. That's why focusing too much effort on one series and one genre at the expense of others isn't working and is sinking the Wii U like a stone.