The ornamental design for a mop holder, as shown and described.
The file of this patent contains at least one drawing/photograph executed in color. Copies of this patent with color drawing(s)/photograph(s) will be provided by the Office upon request and payment of the necessary fee.
FIG. 1 is a front view for my design for a mop holder in a first state of use;
FIG. 2 is a front view of the mop holder in a second state of use;
FIG. 3 is a right-side view of the mop holder in FIG. 1;
FIG. 4 is a left-side view thereof;
FIG. 5 is a rear view thereof;
FIG. 6 is a top view thereof; and,
FIG. 7 is a front, top, right perspective view thereof.
The colors shown form a part of the claimed design. The bottom view forms no part of the claimed design.