She appears in a flashback scene in Episode 2595, where she, Gina and Yyvone try to study for school. Thelma appears to be the most studious, as Gina and Yyvone would rather goof off and snack. They are then roped into Big Bird's demonstration of big, bigger and biggest.
She re-appeared the following season in Episode 2745, where Gina's high school class is graduating. She is seen with their friend group signing each other's yearbooks, and later as one of the first recipients at the graduation ceremony.
She shares her name with Sesame Street script coordinator Thelma Moses.