I think Terezi would be really flattered to know Shepard thinks of her as an adult. She doesn't feel very much like an adult right now. She feels like she's faking it a lot of the time, or trying to do her best to keep up with the real adults.

She's glad that Shepard considers her an ally. She'll never admit it, but she still looks up to Shepard and values her opinion on how she's doing. Spacemom will always be spacemom.