I've ranked the following issues I've found in terms of least problematic to most problematic (1-10). Please feel free to disagree with any of this, it's just my opinion, but I hope it helps.
-(7) I'm not sure if this is just my computer or something, but some of the controls are off key wise. R is interact, T is reset. This doesn't always seem to be the case but it does happen sometimes.
-(6) The four torches in the cave area with the coin seem to flicker between being visible and invisible as their sprites change.
-(5) Title background does not fully scale to screen size, leaving slight blank space on the top and bottom. I would give advice on how to fix this, but I have not fully figured out how to do this either. I've ranked this more problematic because it could be the difference between if someone decides to play or not.
-(4) The music does not loop on levels 1 & 2.
-(3) Occasionally, objects that can be moved are bounced back and forth between the ninja stars, sometimes resulting in the object moving towards you instead of away.
-(3) This could be related to the screen size UI scaling issue, but the plants near the bottom of the screen on the level select on the title screen are duplicated and cut off. I assume the plants are copied onto the tan background sprite as an overlay?
-(3) The time did not seem to save for level 2.
-(2) Personally, I'm fine with Fullscreen, but certain other people may not be, so a Fullscreen option in the settings could be good.
-(2) Grammar error in the title screen hint text "Find three secret chest's hidden within each level."
-(2) The audio of the jumping sound effect seems to lag behind a bit. If you move left and jump, it's on the right side of the headphones for a few seconds before it seems to slowly catch up.
-(1) It is a bit unclear at first that you are required to click the skull to delete all save data in the title screen, especially since it doesn't have an animation.
-(1) The coin count, at least for me, was a bit unclear. I thought it meant I started with 12 coins. Forgive me if this is common, I don't play platformers often.
Most of these issues are quite minor, and can likely be fixed quickly and easily, so I hope this list isn't overwhelming in any way. I did enjoy playing this game, and it's very relaxing atmosphere. Thank you for making this, I hope my feedback helps in it's development. (Feel free to delete this comment whenever, it probably takes up a lot of space)