The ornamental design for a tongue mounted mouthbrush, as shown and described.
FIG. 1 is a front view of the device.
FIG. 2 is a side view of the device.
FIG. 3 is a top view of the device.
FIG. 4 is a bottom view of the device; and,
FIG. 5 is a reduced side view with the broken line showing of a user being included for environmental purposes and forming no part of the claimed design.