Looks alright to me.
I can think of a few things that are off...
The princess would share skin color with Mario, as do Luigi and even Bowser. Enemies like the Koopas, Cheep-Cheeps, and Lakitus also share the same yellow color.
The black eyes look out of place when all the other characters do not have black eyes, except the Goombas and Bullet Bills. But the black is also used for other stuff in that case.
The princess's arms and face are too small compared to Mario's.
Most of the sprites don't have pixels that connect diagonally (like on the crown). The only thing I can think of that do is the Koopa shells.
Also, the hair kinda looks off. Not in relation to Peach's actual hair, but in comparison to the rest of the game. I don't know how to really explain it, though.
To me, it's weird that nearly all the characters face either to the side or at the screen (except Cheep-Cheep and Lakitu, though he's looking down at Mario, so...), except for Peach.
Finally, it looks a little too detailed compared to the rest of the game's simplistic look.
I dunno, maybe it's just me.
