I'm not sure what that graph refer to (can't read japanese) however I'm sure Galaxy began a progressive simplification process which culminated in the 3D Land/World games.
Yeah, thats where the chart is coming from. Starting with Galaxy, folks at Nintendo were consciously aiming for something more immediately accessible, something with a lower 'skill floor' (note that this is not the same thing as a lower 'skill ceiling'), by way of a reduction/elimination of the challenge of managing/directing the camera, as well as a (related) reduction/elimination of the game element of searching (i.e., exploration):
https://www.wired.com/2007/07/e3-interview-ni
Miyamoto: Weve kind of had this idea for a long time, that we wanted to, from the very beginning, have [Mario Galaxy] be something that everyone can play. And the core idea, actually, in terms of the gravity and running around on different spheres, were things that we were experimenting with back when we were working on Mario 128, and the 100 Marios demo, immediately after Mario 64. A lot of it was ideas that weve had for a long time. The main advantage of playing on globes or spheres rather than a big open 3D space is that the camera becomes less of an issue, to always see what youre doing. Its much easier for people who have a hard time with 3D space to navigate...
http://www.eurogamer.net/articles/2013-10-24-touring-the-world-with-miyamoto
"Super Mario 64 was a key turning point in the history of Super Mario," explains Miyamoto (before adding, with typical modesty: "It was a good game but we were not quite able to achieve something that was both fun and easy for both beginners and advanced players.") Miyamoto admits that the Super Mario Galaxy games, which tried to find a better balance for novice and expert, still skewed towards the latter kind of player. "But with Super Mario 3D Land we found the spirit of the original games which were easy for beginners [low skill floor] but allowed for advanced play [high skill ceiling]," he says, "all within the 3D environment."
http://iwataasks.nintendo.com/interviews/#/3ds/super-mario-3d-land/1/0
Koizumi: Early on [in the development of Super Mario 3D Land], we established the theme of making a 3D Super Mario game that would be close to 2D so that anyone could play. One big difference between 2D and 3D is the camera. We made adjustments as we went to the parallel-track camera we used in Super Mario Galaxy 2 so it would be easy to play.
Iwata: A parallel-track camera moves in parallel to Mario at a set distance
Tezuka-san, you were in charge of 2D Super Mario games. The 3D Super Mario games have always been Miyamoto-san's sphere, while you were in a position slightly removed from them. What made you become actively involved this time?
Tezuka: I never said much with regard to the 3D Super Mario games so far, but I sensed there were a lot of 2D Super Mario elements in this game, so I had things to say. I was like, "Let me chime in!" (laughs)
Koizumi: Well
I thought, "At last, we motivated him!" (laughs) This time [with 3D Land], we were putting aside the game element we carried on since Super Mario 64 of searching [i.e., exploration] and were trying to reconstruct the game with the original Super Mario elements. I think perhaps that is why Tezuka-san couldn't help but feel as if it flowed like a 2D Super Mario game...