The design is the visual features of the INSULATING CUP SLEEVE shown in solid lines in the drawings, whether those features are features of one of shape, configuration, ornament or pattern or are a combination of any of these features. The portions shown in broken lines do not form part of the design. The bold, wavy lines do not constitute a feature of the design.Fig. 1 is a perspective view of a design of an INSULATING CUP SLEEVE;Fig. 2 is a side elevational view thereof;Fig. 3 is a top plan view thereof;Fig. 4 is a bottom plan view thereof; andFig. 5 is a perspective view showing the environment in which the design shown in Fig. 1 is used. Drawings of the design are included.