"Also, why not Sonic 3 & Knuckles style?"
That's kind of a question you shouldn't be asking. It seems like you're disregarding the fact that that's maybe the style he
wants to sprite in.
As for my own two bits on this: Mario's nose isn't all that noticeable here. I understand that noses aren't really that noticable in this style, but Mario
has to have that nose - otherwise, he just isn't the same Mario.
Secondly, Mario looks
really skinny here. Sonic only looks that thin because of his build. Mario's portly officially.
Eyes can use a little bit of tweaking, but they're not otherwise very bad.
This is a decent attempt at a custom Sonic 2 styled Mario, though.