I claim the ornamental design for a mitten teether, as shown and described.
FIG. 1 is a front view of the mitten teether showing my new design;
FIG. 2 is a side view thereof;
FIG. 3 is a top view thereof;
FIG. 4 is a bottom view thereof; and,
FIG. 5 is a perspective view of the mitten teether shown on a reduced scale in condition of use with an infant that is shown in broken lines for illustrative purposes only and forming no part of the claimed design.