I'm far from good at pixel art, but I could notice:

- the pixel sizes not being the same (example: hearts in the UI are scaled up a lot compared to let's say the monk and birds, also the clouds were too scaled up)

- inconsistency in outlines (this could be by design since you had gameplay reasons for it, but an outline for the monk (maybe) would've made everything fit together better, I might be wrong on this, but you should usually go for all sprites having outlines or no outlines at all, or mix them when you see fit [for example leave the background and environment with no outlines, but make all characters and Interactables have outlines, you should also check if they fit together])

- just one more thought on the outlines, they should almost always have the same pixel size across the game

- a consistent color palette could help a lot for future projects if you plan on making (or editing sprites you find online), try https://lospec.com/ for finding them, don't try to make your own yet

- watch this video by Juniper Dev if you want a more in depth explanation on how and why pixel art works 

I also wanted to say that I really liked the bird dancing game you made a while ago, that's why I played this one too.