We claim the ornamental design for a balloon cup, as shown and described.
FIG. 1 is a perspective view of a balloon cup showing our new design;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a left side view thereof;
FIG. 5 is a right side view thereof;
FIG. 6 is a top view thereof;
FIG. 7 is a cross-sectional view thereof, taken in the direction of arrows 7-7 in FIG. 6; and,
FIG. 8 is a bottom view thereof.
The broken line depiction of the structure in the FIG. 7 view has been shown for the purpose of illustrating a portion of the balloon cup that forms no part of the claimed design.