The ornamental design for a mop head, as shown and described.
FIG. 1 is a perspective view of a new design for a mop head;
FIG. 2 is a right side view thereof, the left side view being a mirror image thereof;
FIG. 3 is a front view thereof, the rear view being a mirror image thereof;
FIG. 4 is a top view thereof;
FIG. 5 is a bottom view thereof; and,
FIG. 6 is a perspective view thereof of the mop head shown connected with a handle.
The broken lines in the drawings are shown for illustrative purposes only and form no part of the claimed design. In addition, the surface shading illustrated in the drawings is provided merely to highlight the contour of design; it is not intended to be illustrative of texture or gloss.