It sounds a bit overwhelming, as BlockGhost said, there could be some more work put into mixing. But the effort can be seen, thus I'm giving you a high rate :)

You are welcome. ;)
Also, I love undertale, so I'm happy to find a remix 8)

A lot of things were competing to be the main sound. Which means different melodies were odd compared to others, or next to another. You need to learn how to mix better, but that's only a matter of time and listening. Another thing is that you need to stay in your scale. There are times where you can change a scale or add a key not seen in a scale but that's pretty hard to pull off in a general song like this one which is a remix.

So,

-Mixing
-Sound Balance
-Scales
-Not putting every melody in the same place at once