I claim, the ornamental design for a flat mouth outlet pipe baffle of an air conditioner, as shown and described.
FIG. 1 is a front, left perspective view of a flat mouth outlet pipe baffle of an air conditioner showing my new design;
FIG. 2 is a rear, right perspective view thereof;
FIG. 3 is a front view thereof;
FIG. 4 is a rear view thereof;
FIG. 5 is a left view thereof;
FIG. 6 is a right view thereof;
FIG. 7 is a top view thereof; and,
FIG. 8 is a bottom view thereof.
The broken lines shown on the drawings depict portions of the flat mouth outlet pipe baffle of an air conditioner that form no part of the claimed design.