The ornamental design for a tissueboxcover, as shown and described.
FIG. 1 is a front view of a tissue box cover showing our new design;
FIG. 2 is a rear view thereof;
FIG. 3 is a top view thereof;
FIG. 4 is a bottom view thereof;
FIG. 5 is a right view thereof; and,
FIG. 6 is a right perspective view thereof.
The broken-line disclosure in the views is understood to represent portions of the article in which the claimed design is embodied, but which forms no part of the claimed design.