The ornamental design for a tissueboxcover, as shown and described.
FIG. 1 is a top perspective view of a tissue box cover showing our new design.
FIG. 2 is a top plan view thereof.
FIG. 3 is a rear elevational view thereof.
FIG. 4 is a left elevational side view thereof.
FIG. 5 is a right elevational side view thereof.
FIG. 6 is a front elevational view thereof; and,
FIG. 7 is a an additional top perspective view thereof, with the broken-line disclosure of tissue paper being for illustrative purposes only and forming no part of the claimed design.