Well, for one thing, you would need to block out 2% of sunlight to counteract the warming effect that we've seen since the industrial revolution.

But more importantly, you are underestimating the scale. The earth is big, and L1 is far away. A device that blocks 2% of the sunlight at that distance would have to cover 4.5 million square kilometers, a disc that size would be larger than the moon, or about the size of the entire United States including Alaska.

It would also require engines that need to be constantly refueled, because solar wind, which is really strong at the scale we're talking about, would push it away in just a few months.

And the screen would need to be smart, because L1 is only quasi-stable. The actual point where there is zero differential gravity is, well, a point, and any deviation from that point will inevitably increase until the object eventually falls into a regular orbit, so to stay there you need to be constantly monitoring your own orbit and correcting for deviations. Only L4 and L5 are self-correcting (which is why there are no known natural objects at L1-3 of any two-body system)