The ornamental design for the bottom bouncer, as shown and described.
FIG. 1 is a right side perspective view of the bottom bouncer of the present invention;
FIG. 2 is a right side view thereof;
FIG. 3 is a left side view thereof;
FIG. 4 is a front view thereof;
FIG. 5 is a rear view thereof;
FIG. 6 is a top view thereof; and,
FIG. 7 is a bottom view thereof.
The broken (dash-dot) lines denote indicia and form no part of the claimed invention. Dashed lines indicate unclaimed matter and form no part of the invention. Break lines in the figures indicate a symbolic break in length of the device and any portion of the article between the break lines forms no part of the claimed design.