Some generic avatars will not show locally in mirror, but some will. If you have an animator on the root of the Generic avatar, it works and shows. If you do not have an animator on the root of the Generic avatar, it will not show in mirror reflections locally.
Generic Avatars that do not reflect locally:
Generic Avatars that do reflect locally:
Wear one of the avatars listed above.
Go to a world with a mirror.
Observe yourself in the mirror. Optionally, have someone present.
You see yourself in the mirror as expected.
Avatars listed above under "Generic Avatars that do not reflect locally" do not show in the mirror. Oddly, lights that are on the avatars still cast on nearby geometry/avatars.