How to salvage a cookie dough that is too sweet?
It is almost impossible to reduce the sweetness of cookie dough once made, without further reducing the quality.

The closest you could come would be to create a second batch of cookie dough ingredients with reduced sugar, and combine it with your existing dough.  However, it will be difficult to achieve good integration of the two batches, and even if you do, the extra working of the first batch from the mixing will create additional gluten, making tougher cookies.

In this case, you might want to try some sort of tart lemon glaze or perhaps a very bittersweet chocolate icing to compliment your cookies and reduce the impression of sweetness.  On the other hand, except for butter, sugar cookies contain no expensive ingredients.  It may not be worth trying to salvage this batch.

In the end, this is probably a lesson learned that your particular recipe is not to your liking as is.  Next time, you will want to either reduce the sugar, or use another recipe.