I would be cautious about mythologizing Miyamoto's work too much. A lot of these concepts seem obvious now, but only because they've become entrenched over years of reiteration and gradual refinement. These designs more than likely
were the result of pure whimsy, despite the stand-out quotes from interviews on them. You can see how they changed the designs as early as the second game, and with good reason as the original Mario sprite has an awkward palette (red, brown, peach?) and poor contrast. I'm quite certain the reason Mario's design (Italian plumber caricature) is so recognizable is because it was a good game at the right time and it was followed by a successful series and rigorous marketing. Sorry, I just have to get that off my chest because I see the story cited all the time, and I'm really wary of this kind of retrospective legend-making. Besides Miyamoto is still alive, it's too early to canonize him

THAT said, your advice is sound! Really, the thing I think your sprite needs now is a little colour separation. It will depend on the setting in game, of course, but at the moment it looks less "muted" and more "washed out."


Arachne's edit is also quite helpful, I think. At the moment he appears to be leaning forward. You can see that it changes his character quite a bit though, you could even pull a little less of a casual look if you make his legs pretty straight.
The character is really strong though. Very human proportions for this kind of art and it comes across great as an actual, well, person. I know that's not the critique you're asking for with this post, but robotacon and others can give much sounder animation advice than I
