The ornamental design for a pop-up lawn sprinkler, as shown and described.
FIG. 1 is a side view of a pop-up lawn sprinkler showing our new design, and showing in broken outline a replaceable sprinkler head and a garden hose, not parts of the present invention, the unshown opposite side being of identical appearance;
FIG. 2 is a perspective side view thereof;
FIG. 3 is a right end view thereof, showing the closure cap in its closed position;