The mix is too quiet, volume peaks are not big, so you can increase the volume without adding a limiter. The piano is a bit dry (FL Studio has a great built-in grand piano, you can find it in the instruments folder) and in my opinion 8-bit drums would fit better but still, good job. Just some little tweaks and it would sound perfectly fine.
Upd. Judging by the waveform, you most likely have a limiter, but you turned the volume down too much
Upd2. Yep, the piano I'm talking about is a directwave preset, you can find it in FL studio's browser, just type "grand". About mastering and all this stuff, here's a playlist: https://youtube.com/playlist?list=PL5353C9EB8A886421&si=XTeN54SgLOj_10Xn
You can find information about fl's stock plugins and some tutorials there