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 left elevation view, and the right elevation view of the insulating cup sleeve are each an identical image of the view shown in FIG. 3).
The broken lines shown in the drawing views are included for the purpose of showing environmental structure or boundary lines that forms no part of the claimed design.