While it appears there is currently no way to make a member a moderator of an entire category in SMF, you can make the member a moderator of each board within the category.
Click the Admin tab and then going to Forum -> Boards -> Modify boards.
Then click where it says “Modify” next to the board within the category you want to edit.
The next to the “Moderators” field you can type the member’s name you wish to make a moderator of that board. You would then repeat this process for each board within the category, effectively making that member a moderator of the entire category.
Perhaps in a future release the simple machines developers will add this feature, but currently this is the work around.