Honestly this demo is amazing the smooth controls, the good difficulty level, the downright AWESOME soundtrack, and the beautiful artwork. this is something I am going to be looking forward to the coming years. However, I wouldn't be writing this review if it weren't for some flaws and bugs.
For one, some of the hitboxes can be kind of dodgy. but eh, all mario games had this for me at least once.

but onto the real bugs.
if you move around at relative speed, the screen gets de-synced. You can see a part of the stage just teleport a few pixels forward.
Another major flaw is the shell hitboxes. The standing still koopa shell has no enemy hitbox. meaning goombas phase right trough 'em.
The falling from the ceiling buzzy beetle shells have no hitbox at all. You cannot interact with it at all whenever it falls from said ceiling.

one final personal nitpick is that mario is too still when standing still. give him some kind of idle animation.

Yes, that is all. For an alpha demo, this is very polished.