You don’t get any “mitzva credit” for anything your tax dollars go to; whether it be supporting torah, poor people, or anything.
The reason for this is that you did not have any choice in giving it.
An essential element in tzedaka is that choosing to give it.
Not to be annoying, but BB 9a (bottom) disagrees, and says taxes do count towards Schar. (although I don’t see it brought down as Halacha in Rambam, perhaps because there is no Nafka Mina)