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
  '';