https://vincerubinetti.github.io/midi-humanizer/
Source code, readme, report issues, suggest features, etc.:
https://github.com/vincerubinetti/midi-humanizer
Original post on FL Studio forums:
https://forum.image-line.com/viewtopic.php?t=306062

I often want to sequence in notes on the grid, then add some randomness to make them feel more human. A lot of DAWs provide little or no functionality to do that, so I made a web app.
I used this on a recent song and it saved so much time. I've tested it a bit with some FL Studio midis, but not much else, so let me know if you find bugs or edge cases. It relies on a third party package to parse and save the midis (converting them to JSON for the app to work with), so that might be a limiting factor. Exported MIDIs often have quirks, but hopefully they shouldn't affect your ability to use the tool for its purpose.
I don't come to this forum often, so if anyone has an issue, please use the GitHub repo issues and discussions. That's really the best way for me to keep track of things. Or if you don't have a GitHub account and don't want to create one, you could email me from vincentrubinetti.com.