Can I use mascarpone as a substitute for cream cheese?

Yes, that should be fine, but bear in mind that cream cheese has a sharper, subtle cheese taste whereas mascarpone is more or less triple cream, and thus milder. If you want that sharp taste, perhaps add just a little lemon juice with your mascarpone.

Mascarpone will substitute for cream cheese in most respects (except, as Elendil mentioned, the precise taste), however it will not entirely bear the same structural qualities and may lead to your cookie spreading out a bit more than the cream cheese would have (in the same way that applesauce can do the same when substituted for a fat). The varieties (garden-variety supermarkets on three uses) I have encountered are more akin to sour cream in texture, having that whipped rather than packed structure.

Obviously the necessity of these instructions depends on how the Mascarpone incorporates, but to stiffen a Mascarpone for use in a cookie recipe (if it is necesary);

• add in a few teaspoons of flour (bit by bit and stirring to keep it from clumping) until it approaches the sturdiness of cream cheese;
• or mix at 1:1 substitution, then add some additional [sifted flour, baking soda/powder, salt] to get the dough to the proscribed density.