The ornamental design for a mattress accessory, as shown and described.
FIG. 1 is a perspective view of a mattress accessory in accordance with one embodiment;
FIG. 2 is a top view of the mattress accessory of FIG. 1;
FIG. 3 is a first side view of the mattress accessory of FIG. 1;
FIG. 4 is a second side view of the mattress accessory of FIG. 1; and,
FIG. 5 is a top, unattached view of the mattress accessory of FIG. 1.
The broken line drawings of the images, text, and other elements shown in the figure are shown for illustrative purposes only and form no part of the claimed design. In addition, the surface shading, if any, illustrated in the drawings is provided merely to highlight the contour of the design; it is not intended to be illustrative of texture or gloss.
The circular elements on the top of the mattress accessory are shown with a symbolic break in height (See side views). The appearance of any portion of the circular element between the break lines forms no part of the claimed design.