The ornamental design for a thermostat mount, as shown and described.
FIG. 1A is a perspective view of a thermostat mount showing the new design;
FIG. 1B is a front elevation view of the thermostat mount; and,
FIG. 1C is a front elevation view of the thermostat mount with a thermostat.
The thermostat mount includes a plain and unornamented top, bottom, left, right, and rear which are not shown; and
The broken line showing of a thermostat in FIG. 1C is included for the purpose of illustrating environmental matter and forms no part of the claimed design.