The ornamental design for a selftanningmittapplicator, as shown and described.
FIG. 1 is a perspective view of a self tanning mitt applicator showing my design;
FIG. 2 is a front view thereof;
FIG. 3 is a back view thereof;
FIG. 4 is a left side view thereof;
FIG. 5 is a right side view thereof;
FIG. 6 is a top view thereof; and,
FIG. 7 is a bottom view thereof.
The broken lines shown in the drawings depict portions of the self tanning mitt applicator in which the design is embodied that form no part of the claimed design.