The ornamental design of a lawnmower accessory, as shown and described.
FIG. 1 is a side view of the lawn mower accessory showing our new design in an arrangement of multiple parts, in use condition.
FIG. 2 is an enlarged side view thereof of the lawnmower accessory.
FIG. 3 is an enlarged perspective view thereof shown in use condition.
FIG. 4 is a top view thereof; and,
FIG. 5 is a front view thereof.
The broken line showing of the cut away wheel structure in FIGS. 1 and 3, the bolts, and other elements in all the figures, is included for the purpose of illustrating environmental structure only and forms no part of the claimed design.