In my honest opinion, animating it just seems like a pain. I would have get a screen recording software. Im looking at this free one calling camstudio right now, but I don't really know if it's good or not. Any who I would record with a green background and you can just key it out.
That would give you much better mouse animation and the fraction of the time trying to animate it. if you want you can actually turn your whole desktop green.
actually, motion sketch is basically screen capturing, it's just recording as keyframe data.
you might try adding the 'smoother' (window>smoother) to try and smooth the keyframe data out some. smoother is destructive (meaning it actually modifies the keyframe data), so you might want to make a copy of the motion sketch layer prior to using smoother on it.
there is also an expression (if you like expressions). the nice thing about the expression is that it isn't destructive, and since it won't change the actual values of the keyframes, you can tweak and tweak until you get it right.
here it is:
just paste that into the expression field for the keyframed property that you want to smooth.
the first value is the time sample that you want it to look at in seconds. '.5' is half a second. the second value is the number of samples to generate for the time sample... for starters you can try the frame rate of the comp and increase if the movement seems jerky as you adjust the first value.