I have a ton of issues with Mario... being kiddy is not one of them.
The story in Mario Sunshine was decent, the animations in the latest Mario games are charming and a delight to look at.
Super Mario RPG and Paper Mario: The Thousand Year Door are the most childish and immature Mario games ever. They are good, really good. The humor, the characters, everything fits perfectly into the universe.
How do you think they fair with games like Journey or Shadow of the COlossus, or even Disney movies like Aladdin or the Lion King?
In a multi-billion dollar industry, and as the icon of the most frugal company within, do you think that difference, if any, is acceptable?
Why would you even compare them with SotC or Journey? They are totally different experiences that should not be compared.
TTYD is a lighthearted, charming story, it has it's radical twists and a very engaging narrative, characters don't need to have a dark background to be interesting. The characters and story in TTYD are excellent, they are not very mature, but they have a very distinctive charm to them, a charm that will make you follow their story with a strong passion. Just think about Vivian's relationship with her sisters, Bobery's backstory, Koops becoming a more mature turtle, Ms. Mowz mysterious personality and Tec's development as a machine who loves and feels.
In Mario RPG, characters are also very funny and interesting, Geno has a very great design and an amazing determination, Mallow has the greatest development of them all. And as simple as the story is, it's really fun to follow.