Man, Grasstown was on my list to remix next after final exams are over and done with.
I definitely agree with DesertDweller here, you need to humanize the recording (tempo variation, DYNAMICS variation, reverb, articulation variation). The music as you have it here is a "flat line" in terms of tension and release, throughout.
On the waltz style: you need more "bounce." Definitely accent those left hand low notes on beat one, and if you have the time, pedal the first to second beat (I know this is tedious), or add just enough reverb to cover it.
Quick note on the fastest moving triplet chords in the right hand: They are impossible. There's no way to program what you're currently doing without the end result sounding sequenced. I would keep the highest notes (the melody) as quick triplets, but remove the harmonies below them, or alternatively keep the sixth below while you play the triplet figure above. If you could e-mail or pm me the midi I could show you what I mean more explicitly.
What really gets me here is the lack of dynamic variation. While Pixel could only do so much in terms of dynamics with his ORG format, he changed up the instruments and noise track enough to provide variation. Just before the halfway point, you have long chords in the right hand that just BEG to be played softer while you give the left hand some neat stuff to mess around with. I should probably think about bringing out the left hand more as a whole.
As a last tidbit, I'd definitely add in some original material or new harmonies. I'd also try and make the ending less abrupt, if you get a chance.
I hate how this whole post sounds like I didn't enjoy your mix, which I did. I'm a huge fan of Cave Story, and I really liked this fresh take on Grasstown. Best of luck!