She has wrinkly bluish-teal skin with light purple lips and wears an orange dress. She also has a small fruit bowl in her gray hair. Her eyes also appear to look in two different directions rather than one and different oval shapes.
SpongeBob pretends to be a mirror by standing in a mirror frame and shapeshifting himself so he looks like whoever comes up to it. When the Ugly Fish comes up to the mirror, she dislikes her "reflection." She sees how ugly she is and knocks out SpongeBob.
Due to her knocking out SpongeBob because of her appearance, the Ugly Fish may not know what she really looks like herself, or is oblivious to her look.