That depends on how hard you pull. It could very well be that the net force in the y direction remains zero. (Note that the normal force of the floor on the box will adjust itself as the angled force is applied.)when you pull a box at an angle is it accelerating in the y direction? I would think it is since there is now an unbalanced force in the y direction.
If this is a typical textbook problem where a box is being pull along a horizontal surface by some rope at an angle, you can most likely assume that the box remains in contact with the surface and that there's no vertical acceleration.its a 2D problem and no the force is at an angle to the plane. its being pulled along the x axis.