The ornamental design for barbecue thermometer, as shown and described.
FIG. 1 is a front, right and top perspective view of barbecue thermometer, showing our design.
FIG. 2 is a front elevation view thereof.
FIG. 3 is a rear elevation view thereof.
FIG. 4 is a left side elevation view thereof.
FIG. 5 is a right side elevation view thereof.
FIG. 6 is a top plan view thereof; and,
FIG. 7 is a bottom plan view thereof.
The broken lines shown in the drawings are included for the purpose of illustrating portions of the barbecue thermometer that form no part of the claimed design.