On the bright side, it's mostly within the limits
... but on the down side, it limits itself a little too much. Why no Triangle love? Needs some triangle wave in there :(
I think the mix could use a bit more balance in the mixing. Keep things within a pretty close level, and make sure the sound doesn't hit the ceiling too hard (which happens here quite often on the main track). A slight nitpick, but the instrument combination on 0:22 goes against NES conventions (2 rectangle, 1 triangle, 1 noise, 1 PCM). I know not everyone goes for the true NES limitations, so I won't knock you for that.
It's a decent effort. Keep on truckin' and you'll get the hang of it.