The ornamental design for this tabletop bag hanger, as shown.
FIG. 1 is a pictorial view of a tabletop bag hanger showing my new design;
FIG. 2 is a top view thereof
FIG. 3 is a front view thereof; and,
FIG. 4 is a side view thereof.
The broken lines showing a portion of a table top and a handbag are for the purposes of illustrating environment and form no part of the claimed design.