The ornamental design for a mouthpiece, as shown and described.
FIG. 1 is a top, front, and left side perspective view of a mouthpiece showing my new design;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a rear elevational view thereof;
FIG. 4 is a left side elevational view thereof;
FIG. 5 is a right side elevational view thereof;
FIG. 6 is a top view thereof; and,
FIG. 7 is a bottom view thereof.
The diagonal short curved lines shown in sets of five represent transparent shading on the exterior surface of the mouthpiece. The uneven-length broken lines shown in the figures represent contour shade lines along the outer transparent shell of the mouthpiece.