The ornamental design for a lighttherapymask, as shown and described.
FIG. 1 is a perspective view of a light therapy mask;
FIG. 2 is a side plan view thereof;
FIG. 3 is a front plan view thereof;
FIG. 4 is a rear plan view thereof;
FIG. 5 is a top end view thereof; and,
FIG. 6 is a bottom end view thereof.
The broken lines in the drawings are included for the purpose of illustrating portions of the light therapy mask that form no part of the claimed design.