It is mostly an optical illusion. If you animate the player and change things to more normal color the "jankiness look" disappear. But retro games are drawn at integer pixels so they can look choppy with really small sprites so you have to apply some tricks to make it look more smooth.
Here I just animated your sprite and changed colors and it looks better straight away:
The more you scale the less it flickers as there are more pixels to draw around: