I guess this whole topic depends on what you mean by "Mario games"... it seems that people are unhappy with the "cuteness" of recent Mario games (there is no such word as "kiddish", folks).
Some recent Mario spin-off games have been extremely "cute" -- Yoshi's Story comes to mind. On the other hand, Mario Golf and Super Smash Bros. (which I see as a Mario spin-off) have a mixture of graphical styles. Mario Golf is, in fact, quite realistic, but with cartoon-style characters. The last _real_ Mario game we've had is Super Mario 64, which was hardly in the same genre as Super Mario Bros./World.
Mario games are designed in a certain style, with characters that are supposed to _look_ like they are animated. That, in and of itself, does not result in an "overly cute" game -- if it did, all animated films would be "kids' stuff", which is insane. Mario isn't a realistic human, but a cartoon character of sorts.
Mario games may well become more realistic, and thus may change in design somewhat. For some examples of more mature level design, take a look at the Ghost House level of Super Mario 64; it's a typical Mario game level, but it is designed with a much moodier atmosphere. Most of the Bowser levels are also much more "moody". The final Bowser battle in SMW2: Yoshi's Island is NOT overly cute, but even somewhat dramatic. The only "overly cute" game I've seen yet is Yoshi's Island, which seems to have been designed with that goal in mind.
Anyway... there is NO such word as "kiddish", nor "funner". Besides, even if a game seems "too cute" for you, what does it matter as long as the game itself is still FUN? That's the mistake many make... "if it isn't graphically violent, dark, moody, and filled with sexual overtones, it isn't any good"; the same thing happened to movies in the late 60's through today, and the same mistake many gamers made in purchasing the PSX.