The ornamental design for a mouthpiececover, as shown and described.
FIG. 1 is a front perspective view of a mouthpiece cover of our design.
FIG. 2 is a bottom perspective view of the mouthpiece cover of FIG. 1.
FIG. 3 is a top view of the mouthpiece cover of FIG. 1.
FIG. 4 is a bottom view of the mouthpiece cover of FIG. 1.
FIG. 5 is a first side view of the mouthpiece cover of FIG. 1.
FIG. 6 is a second side view of the mouthpiece cover of FIG. 1.
FIG. 7 is a rear view of the mouthpiece cover of FIG. 1; and,
FIG. 8 is a front view of the mouthpiece cover of FIG. 1.
Broken lines shown in the figures are included for the purpose of illustrating portions of the mouthpiece cover and form no part of the claimed design.