The ornamental design for a disposable placemat, as shown and described.
FIG. 1 is a top plan view of a disposable placemat showing my new design,
the bottom, not shown, being flat, plain and unornamented; and,
FIG. 2 is a perspective view thereof illustrating the mat attached to a
roll, shown in broken-lines, said roll being environmental and forming no
part of the claimed design.