The ornamental design for a skateboard accessory, as shown and described.
FIG. 1 is a top perspective view of a skateboard accessory showing my new design;
FIG. 2 is a bottom perspective view thereof;
FIG. 3 is a top view thereof;
FIG. 4 is a left side view thereof;
FIG. 5 is a bottom view thereof;
FIG. 6 is a right side view thereof;
FIG. 7 is a front view thereof; and,
FIG. 8 is a rear view thereof.
The structural elements in the figures depicted by means of broken lines have been shown for the purposes of illustrating portions of the skateboard accessory which form no part of the claimed design.