The ornamental design for an insulating cupsleeve, as shown and described.
FIG. 1 is a top perspective view of the design of an insulating cup sleeve showing the new design;
FIG. 2 is a bottom perspective view of the insulating cup sleeve shown in FIG. 1; and,
FIG. 3 is a front elevation view of the insulating cup sleeve shown in FIG. 1 (the rear elevation view, the right side elevation view, and the left side elevation view of the insulating cup sleeve are each an identical image of the view shown in FIG. 3).
The features shown in broken lines depict environmental subject matter only and form no part of the claimed design.