Xorg
To disable mouse acceleration for a touchpad, it's as simple as
services.xserver.libinput.enable = true; services.xserver.libinput.accelProfile = "flat";
But to disable it for a mouse, it's a bit more wordy -
services.xserver.libinput.enable = true; services.xserver.config = '' Section "InputClass" Identifier "mouse accel" Driver "libinput" MatchIsPointer "on" Option "AccelProfile" "flat" Option "AccelSpeed" "0" EndSection '';