An ornamentaldesign for a hat accessory, as shown and described.
FIG. 1 is a front perspective view of a hat accessory, wherein an ornament divided into four pieces is shown attached to a hatband, according to the first embodiment;
FIG. 2 is a side elevation view thereof;
FIG. 3 is a front perspective view of a hat accessory, wherein an ornament divided into three pieces is shown attached to a hatband, according to the second embodiment; and,
FIG. 4 is a side elevation view thereof.
The hat depicted by broken lines represents environmental structure that forms no part of the claimed design.