The ornamental design for a mouthpiece, as shown and described.
FIG. 1 is a perspective view of a mouthpiece showing my new design;
FIG. 2 is a left side elevation view thereof;
FIG. 3 is a right side elevation view thereof;
FIG. 4 is a front elevation view thereof;
FIG. 5 is a rear elevation view thereof;
FIG. 6 is a top plan view thereof; and,
FIG. 7 is a bottom plan view thereof.
The broken lines shown in the drawings depict environmental structure that forms no part of the claimed invention. The appearance of any portion of the article between the break lines forms no part of the claimed design.