We claim the ornamental design for a mobile stand assembly, as shown and described.
FIG. 1 is a top left front perspective view of a mobile stand assembly showing our new design;
FIG. 2 is a front view thereof;
FIG. 3 is a back view thereof;
FIG. 4 is a right side view thereof;
FIG. 5 is a left side view thereof;
FIG. 6 is a top plan view thereof, showing without the broken lines intruded upon the design in order to depict the inner structure of the mobile stand assembly; and,
FIG. 7 is a bottom plan view thereof, showing without the broken lines intruded upon the design in order to depict the inner structure of the mobile stand assembly.
The dashed broken lines illustrate portions of the mobile stand assembly and form no part of the claimed design.