I have seen people complaining that it is a tad too childish, almost always mentioning the colorful visuals and Clown Man's stage.
I agree to an extent, though that doesn't really bothers me.
I think the main weakness of MM8 is how most stages have gimmicks that don't necessarily mix well with the main action we're used to in MM games. I mean things like the lava sailing and the small "puzzles" in Sword Man's stage, the hazard boxes in Clown Man's, the teleport maze in Astro Man's and the absurd difficult spike thanks to the board riding on the first Wily Stage in an otherwise easy game.
Doesn't help that the level design generally isn't that great even when you set those gimmicks aside. Look at Sword Man's stage again; the entire place is basically just the same weak-ass bats and statue-like enemies over and over.
The boss battles are also fairly formulaic. We hardly see battles where the player needs to rely heavily on their own reflexes (think bosses like Quick Man) rather than just spot a pattern of incredibly predictable attacks to win (I admit MM7 is also guilty of this, even though I love that game's bosses).
Combine these and you have the formula for a trivially easy gameplay and broken pacing.
Not that I dislike MM8 or anything. I still think it's a decent game overall and have fun playing it. I can understand why some people bash it, though.