If your using the normal thing you could try:
Event: Every tick/ action set camera X to lerp(camera.X, mouse.X, 0.1)
action set camera Y to lerp(camera.Y, mouse.Y, 0.1)
The 0.1 at the end is the speed so you can adjust that to whatever you want.
I don't know if that will work for you or not, but you could just try it!