We claim the ornamental design for a countertop appliance, as shown and described.
FIG. 1 is a front perspective view of a countertop appliance, showing our new design;
FIG. 2 is a front elevation view thereof;
FIG. 3 is a rear elevation view thereof;
FIG. 4 is a right side elevation view thereof;
FIG. 5 is a left side elevation view thereof;
FIG. 6 is a top plan view thereof; and,
FIG. 7 is a bottom plan view thereof.
The even spaced broken lines in the figure drawings depict portions of the countertop appliance that form no part of the claimed design. The dash-dot broken lines in the figure drawings define the bounds of the claimed design and form no part thereof. The countertop appliance is shown with a symbolic break in its length. The appearance of any portion of the article between the break lines forms no part of the claimed design.